Adds git history squash <revision-range> to fold a range of commits.

Changes in v7:

 * --reedit-message now builds the same editor template git rebase -i shows
   for a squash (a combination of N commits banner with each folded message
   under its own header) and follows autosquash for markers: a fixup!
   message falls out (commented under a will be skipped header), while a
   squash! or amend! keeps its body with only the marker subject commented
   so its remark can be reworded in. Only the message text is affected,
   every commit's changes are always folded in.
 * Reuse git rebase -i's squash-message code: a preparatory sequencer:
   commit extracts the banner, header and marker-comment helpers so both
   rebase and git history squash build the identical template from one
   source.
 * Refuse a range whose oldest commit is a fixup!, squash! or amend!, since
   the marker's target cannot be inside the range.
 * Reorder the squash usage so dashed options come before <revision-range>,
   and spell out HEAD instead of @ in the documentation and examples.
 * Expand the squash commit message and documentation with this overview,
   and scope the merge limitation so it no longer contradicts squash folding
   a single-base interior merge.

Changes in v6:

 * git history squash now accepts multiple revision arguments, read like the
   arguments to git-rev-list, so a compound range such as @~3.. ^topic
   works.
 * The base to reparent onto is now the oldest in-range commit's parent; a
   boundary other than that base means the range has more than one base and
   is rejected. This also fixes the earlier overly-restrictive handling of
   merges and side branches.
 * A single-commit range (e.g. @^!) is rejected with "nothing to squash"
   (this also covers the @^!-style example that previously succeeded
   silently).
 * Commit messages reworded: the squash commit now gives an overview of
   fixup!/squash!/amend! handling, rewording, merge-parent and ref behavior.

Changes in v5:

 * The range walk now uses --ancestry-path, so only commits descended from
   the base are folded; a single revision such as HEAD or HEAD~1 is now
   rejected as "not a <base>..<tip> range" rather than treated as a squash
   down to the root.
 * This adopts the --ancestry-path suggestion; the multi-base rejection is
   unchanged, so a side branch that forked before the base and merged in is
   still refused.
 * Added tests covering more merge topologies: two interior merges, a nested
   merge, an octopus merge, an octopus arm forked before the base, a merge
   among the descendants replayed above the range, and a ref pointing at an
   interior merge commit.

Changes in v4:

 * git history squash now detects when another ref points at a commit inside
   the range being folded and refuses, with an advice.historyUpdateRefs hint
   to use --update-refs=head.
 * A merge inside the range is folded fine as long as the range has a single
   base; a range with merge commit at the tip or base also folds correctly.
   Only a range with more than one base is rejected.

Changes in v3:

 * Moved the feature out of git rebase and into a new git history squash
   <revision-range> subcommand, per the list discussion. git rebase --squash
   is dropped.
 * Takes an arbitrary range (git history squash @~3.., git history squash
   @~5..@~2), folding it into the oldest commit and replaying any
   descendants on top.
 * Implemented as a single tree operation rather than picking each commit,
   so there are no repeated conflict stops (addresses Phillip's efficiency
   point).
 * A merge inside the range is folded fine, only a range with more than one
   base is rejected.
 * --reedit-message seeds the editor with every folded-in message, not just
   the oldest.

Harald Nordgren (5):
  history: extract helper for a commit's parent tree
  history: give commit_tree_ext a message template
  history: add squash subcommand to fold a range
  sequencer: extract helpers for the squash message markers
  history: re-edit a squash with every message

 Documentation/config/advice.adoc |   4 +
 Documentation/git-history.adoc   |  49 ++-
 advice.c                         |   1 +
 advice.h                         |   1 +
 builtin/history.c                | 390 +++++++++++++++++--
 sequencer.c                      |  64 ++--
 sequencer.h                      |  23 ++
 t/meson.build                    |   1 +
 t/t3455-history-squash.sh        | 632 +++++++++++++++++++++++++++++++
 9 files changed, 1099 insertions(+), 66 deletions(-)
 create mode 100755 t/t3455-history-squash.sh

base-commit: e9019fcafe0040228b8631c30f97ae1adb61bcdc

Submitted-As: https://lore.kernel.org/git/pull.2337.v7.git.git.1783327849.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.2337.git.git.1781465141.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.2337.v2.git.git.1781512625.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.2337.v3.git.git.1781810226.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.2337.v4.git.git.1782021195.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.2337.v5.git.git.1782338102.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.2337.v6.git.git.1782635349.gitgitgadget@gmail.com