checkout: split part of it to new command 'switch'

"git checkout" doing too many things is a source of confusion for many
users (and it even bites old timers sometimes). To remedy that, the
command will be split into two new ones: switch and restore. The good
old "git checkout" command is still here and will be until all (or most
of users) are sick of it.

See the new man page for the final design of switch. The actual
implementation though is still pretty much the same as "git checkout"
and not completely aligned with the man page. Following patches will
adjust their behavior to match the man page.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.gitignore

@@ -167,6 +167,7 @@
 /git-submodule
 /git-submodule--helper
 /git-svn
+/git-switch
 /git-symbolic-ref
 /git-tag
 /git-unpack-file

Documentation/config/advice.txt

@@ -42,7 +42,8 @@ advice.*::
 		state in the output of linkgit:git-status[1], in
 		the template shown when writing commit messages in
 		linkgit:git-commit[1], and in the help message shown
-		by linkgit:git-checkout[1] when switching branch.
+		by linkgit:git-switch[1] or
+		linkgit:git-checkout[1] when switching branch.
 	statusUoption::
 		Advise to consider using the `-u` option to linkgit:git-status[1]
 		when the command takes more than 2 seconds to enumerate untracked
@@ -62,12 +63,14 @@ advice.*::
 		your information is guessed from the system username and
 		domain name.
 	detachedHead::
-		Advice shown when you used linkgit:git-checkout[1] to
-		move to the detach HEAD state, to instruct how to create
-		a local branch after the fact.
+		Advice shown when you used
+		linkgit:git-switch[1] or linkgit:git-checkout[1]
+		to move to the detach HEAD state, to instruct how to
+		create a local branch after the fact.
 	checkoutAmbiguousRemoteBranchName::
 		Advice shown when the argument to
-		linkgit:git-checkout[1] ambiguously resolves to a
+		linkgit:git-checkout[1] and linkgit:git-switch[1]
+		ambiguously resolves to a
 		remote tracking branch on more than one remote in
 		situations where an unambiguous argument would have
 		otherwise caused a remote-tracking branch to be

Documentation/config/branch.txt

@@ -1,5 +1,5 @@
 branch.autoSetupMerge::
-	Tells 'git branch' and 'git checkout' to set up new branches
+	Tells 'git branch', 'git switch' and 'git checkout' to set up new branches
 	so that linkgit:git-pull[1] will appropriately merge from the
 	starting point branch. Note that even if this option is not set,
 	this behavior can be chosen per-branch using the `--track`
@@ -11,7 +11,7 @@ branch.autoSetupMerge::
 	branch. This option defaults to true.
 
 branch.autoSetupRebase::
-	When a new branch is created with 'git branch' or 'git checkout'
+	When a new branch is created with 'git branch', 'git switch' or 'git checkout'
 	that tracks another branch, this variable tells Git to set
 	up pull to rebase instead of merge (see "branch.<name>.rebase").
 	When `never`, rebase is never automatically set to true.

Documentation/config/checkout.txt

@@ -1,5 +1,6 @@
 checkout.defaultRemote::
-	When you run 'git checkout <something>' and only have one
+	When you run 'git checkout <something>'
+	or 'git switch <something>' and only have one
 	remote, it may implicitly fall back on checking out and
 	tracking e.g. 'origin/<something>'. This stops working as soon
 	as you have more than one remote with a '<something>'
@@ -8,8 +9,10 @@ checkout.defaultRemote::
 	disambiguation. The typical use-case is to set this to
 	`origin`.
 +
-Currently this is used by linkgit:git-checkout[1] when 'git checkout
-<something>' will checkout the '<something>' branch on another remote,
+Currently this is used by linkgit:git-switch[1] and
+linkgit:git-checkout[1] when 'git checkout <something>'
+or 'git switch <something>'
+will checkout the '<something>' branch on another remote,
 and by linkgit:git-worktree[1] when 'git worktree add' refers to a
 remote branch. This setting might be used for other checkout-like
 commands or functionality in the future.

Documentation/config/diff.txt

@@ -78,7 +78,8 @@ diff.external::
 diff.ignoreSubmodules::
 	Sets the default value of --ignore-submodules. Note that this
 	affects only 'git diff' Porcelain, and not lower level 'diff'
-	commands such as 'git diff-files'. 'git checkout' also honors
+	commands such as 'git diff-files'. 'git checkout'
+	and 'git switch' also honor
 	this setting when reporting uncommitted changes. Setting it to
 	'all' disables the submodule summary normally shown by 'git commit'
 	and 'git status' when `status.submoduleSummary` is set unless it is

Documentation/git-checkout.txt

@@ -560,6 +560,10 @@ $ edit frotz
 $ git add frotz
 ------------
 
+SEE ALSO
+--------
+linkgit:git-switch[1]
+
 GIT
 ---
 Part of the linkgit:git[1] suite

Documentation/git-switch.txt

@@ -0,0 +1,271 @@
+git-switch(1)
+=============
+
+NAME
+----
+git-switch - Switch branches
+
+SYNOPSIS
+--------
+[verse]
+'git switch' [<options>] [--no-guess] <branch>
+'git switch' [<options>] --detach [<start-point>]
+'git switch' [<options>] (-c|-C) <new-branch> [<start-point>]
+'git switch' [<options>] --orphan <new-branch>
+
+DESCRIPTION
+-----------
+Switch to a specified branch. The working tree and the index are
+updated to match the branch. All new commits will be added to the tip
+of this branch.
+
+Optionally a new branch could be created with either `-c`, `-C`,
+automatically from a remote branch of same name (see `--guess`), or
+detach the working tree from any branch with `--detach`, along with
+switching.
+
+Switching branches does not require a clean index and working tree
+(i.e. no differences compared to `HEAD`). The operation is aborted
+however if the operation leads to loss of local changes, unless told
+otherwise with `--discard-changes` or `--merge`.
+
+OPTIONS
+-------
+<branch>::
+	Branch to switch to.
+
+<new-branch>::
+	Name for the new branch.
+
+<start-point>::
+	The starting point for the new branch. Specifying a
+	`<start-point>` allows you to create a branch based on some
+	other point in history than where HEAD currently points. (Or,
+	in the case of `--detach`, allows you to inspect and detach
+	from some other point.)
++
+You can use the `@{-N}` syntax to refer to the N-th last
+branch/commit switched to using "git switch" or "git checkout"
+operation. You may also specify `-` which is synonymous to `@{-1}`.
+This is often used to switch quickly between two branches, or to undo
+a branch switch by mistake.
++
+As a special case, you may use `A...B` as a shortcut for the merge
+base of `A` and `B` if there is exactly one merge base. You can leave
+out at most one of `A` and `B`, in which case it defaults to `HEAD`.
+
+-c <new-branch>::
+--create <new-branch>::
+	Create a new branch named `<new-branch>` starting at
+	`<start-point>` before switching to the branch. This is a
+	convenient shortcut for:
++
+------------
+$ git branch <new-branch>
+$ git switch <new-branch>
+------------
+
+-C <new-branch>::
+--force-create <new-branch>::
+	Similar to `--create` except that if `<new-branch>` already
+	exists, it will be reset to `<start-point>`. This is a
+	convenient shortcut for:
++
+------------
+$ git branch -f <new-branch>
+$ git switch <new-branch>
+------------
+
+-d::
+--detach::
+	Switch to a commit for inspection and discardable
+	experiments. See the "DETACHED HEAD" section in
+	linkgit:git-checkout[1] for details.
+
+--guess::
+--no-guess::
+	If `<branch>` is not found but there does exist a tracking
+	branch in exactly one remote (call it `<remote>`) with a
+	matching name, treat as equivalent to
++
+------------
+$ git switch -c <branch> --track <remote>/<branch>
+------------
++
+If the branch exists in multiple remotes and one of them is named by
+the `checkout.defaultRemote` configuration variable, we'll use that
+one for the purposes of disambiguation, even if the `<branch>` isn't
+unique across all remotes. Set it to e.g. `checkout.defaultRemote=origin`
+to always checkout remote branches from there if `<branch>` is
+ambiguous but exists on the 'origin' remote. See also
+`checkout.defaultRemote` in linkgit:git-config[1].
++
+`--guess` is the default behavior. Use `--no-guess` to disable it.
+
+-f::
+--force::
+	An alias for `--discard-changes`.
+
+--discard-changes::
+	Proceed even if the index or the working tree differs from
+	`HEAD`. Both the index and working tree are restored to match
+	the switching target. If `--recurse-submodules` is specified,
+	submodule content is also restored to match the switching
+	target. This is used to throw away local changes.
+
+-m::
+--merge::
+	If you have local modifications to one or more files that are
+	different between the current branch and the branch to which
+	you are switching, the command refuses to switch branches in
+	order to preserve your modifications in context.  However,
+	with this option, a three-way merge between the current
+	branch, your working tree contents, and the new branch is
+	done, and you will be on the new branch.
++
+When a merge conflict happens, the index entries for conflicting
+paths are left unmerged, and you need to resolve the conflicts
+and mark the resolved paths with `git add` (or `git rm` if the merge
+should result in deletion of the path).
+
+--conflict=<style>::
+	The same as `--merge` option above, but changes the way the
+	conflicting hunks are presented, overriding the
+	`merge.conflictStyle` configuration variable.  Possible values are
+	"merge" (default) and "diff3" (in addition to what is shown by
+	"merge" style, shows the original contents).
+
+-q::
+--quiet::
+	Quiet, suppress feedback messages.
+
+--progress::
+--no-progress::
+	Progress status is reported on the standard error stream
+	by default when it is attached to a terminal, unless `--quiet`
+	is specified. This flag enables progress reporting even if not
+	attached to a terminal, regardless of `--quiet`.
+
+-t::
+--track::
+	When creating a new branch, set up "upstream" configuration.
+	`-c` is implied. See `--track` in linkgit:git-branch[1] for
+	details.
++
+If no `-c` option is given, the name of the new branch will be derived
+from the remote-tracking branch, by looking at the local part of the
+refspec configured for the corresponding remote, and then stripping
+the initial part up to the "*".  This would tell us to use `hack` as
+the local branch when branching off of `origin/hack` (or
+`remotes/origin/hack`, or even `refs/remotes/origin/hack`).  If the
+given name has no slash, or the above guessing results in an empty
+name, the guessing is aborted.  You can explicitly give a name with
+`-c` in such a case.
+
+--no-track::
+	Do not set up "upstream" configuration, even if the
+	`branch.autoSetupMerge` configuration variable is true.
+
+--orphan <new-branch>::
+	Create a new 'orphan' branch, named `<new-branch>`. All
+	tracked files are removed.
+
+--ignore-other-worktrees::
+	`git switch` refuses when the wanted ref is already
+	checked out by another worktree. This option makes it check
+	the ref out anyway. In other words, the ref can be held by
+	more than one worktree.
+
+--recurse-submodules::
+--no-recurse-submodules::
+	Using `--recurse-submodules` will update the content of all
+	initialized submodules according to the commit recorded in the
+	superproject. If nothing (or `--no-recurse-submodules`) is
+	used, the work trees of submodules will not be updated. Just
+	like linkgit:git-submodule[1], this will detach `HEAD` of the
+	submodules.
+
+EXAMPLES
+--------
+
+The following command switches to the "master" branch:
+
+------------
+$ git switch master
+------------
+
+After working in the wrong branch, switching to the correct branch
+would be done using:
+
+------------
+$ git switch mytopic
+------------
+
+However, your "wrong" branch and correct "mytopic" branch may differ
+in files that you have modified locally, in which case the above
+switch would fail like this:
+
+------------
+$ git switch mytopic
+error: You have local changes to 'frotz'; not switching branches.
+------------
+
+You can give the `-m` flag to the command, which would try a three-way
+merge:
+
+------------
+$ git switch -m mytopic
+Auto-merging frotz
+------------
+
+After this three-way merge, the local modifications are _not_
+registered in your index file, so `git diff` would show you what
+changes you made since the tip of the new branch.
+
+To switch back to the previous branch before we switched to mytopic
+(i.e. "master" branch):
+
+------------
+$ git switch -
+------------
+
+You can grow a new branch from any commit. For example, switch to
+"HEAD~3" and create branch "fixup":
+
+------------
+$ git switch -c fixup HEAD~3
+Switched to a new branch 'fixup'
+------------
+
+If you want to start a new branch from a remote branch of the same
+name:
+
+------------
+$ git switch new-topic
+Branch 'new-topic' set up to track remote branch 'new-topic' from 'origin'
+Switched to a new branch 'new-topic'
+------------
+
+To check out commit `HEAD~3` for temporary inspection or experiment
+without creating a new branch:
+
+------------
+$ git switch --detach HEAD~3
+HEAD is now at 9fc9555312 Merge branch 'cc/shared-index-permbits'
+------------
+
+If it turns out whatever you have done is worth keeping, you can
+always create a new name for it (without switching away):
+
+------------
+$ git switch -c good-surprises
+------------
+
+SEE ALSO
+--------
+linkgit:git-checkout[1],
+linkgit:git-branch[1]
+
+GIT
+---
+Part of the linkgit:git[1] suite

Documentation/gitattributes.txt

@@ -112,7 +112,8 @@ Checking-out and checking-in
 
 These attributes affect how the contents stored in the
 repository are copied to the working tree files when commands
-such as 'git checkout' and 'git merge' run.  They also affect how
+such as 'git switch', 'git checkout'  and 'git merge' run.
+They also affect how
 Git stores the contents you prepare in the working tree in the
 repository upon 'git add' and 'git commit'.