Sync with maint

* maint:
  Start preparing for 2.5.1

Documentation/RelNotes/2.5.1.txt

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+Git v2.5.1 Release Notes
+========================
+
+Fixes since v2.5
+----------------
+
+ * Running an aliased command from a subdirectory when the .git thing
+   in the working tree is a gitfile pointing elsewhere did not work.
+
+ * Often a fast-import stream builds a new commit on top of the
+   previous commit it built, and it often unconditionally emits a
+   "from" command to specify the first parent, which can be omitted in
+   such a case.  This caused fast-import to forget the tree of the
+   previous commit and then re-read it from scratch, which was
+   inefficient.  Optimize for this common case.
+
+ * The "rev-parse --parseopt" mode parsed the option specification
+   and the argument hint in a strange way to allow '=' and other
+   special characters in the option name while forbidding them from
+   the argument hint.  This made it impossible to define an option
+   like "--pair <key>=<value>" with "pair=key=value" specification,
+   which instead would have defined a "--pair=key <value>" option.
+
+ * A "rebase" replays changes of the local branch on top of something
+   else, as such they are placed in stage #3 and referred to as
+   "theirs", while the changes in the new base, typically a foreign
+   work, are placed in stage #2 and referred to as "ours".  Clarify
+   the "checkout --ours/--theirs".
+
+ * An experimental "untracked cache" feature used uname(2) in a
+   slightly unportable way.
+
+ * "sparse checkout" misbehaved for a path that is excluded from the
+   checkout when switching between branches that differ at the path.
+
+ * The low-level "git send-pack" did not honor 'user.signingkey'
+   configuration variable when sending a signed-push.
+
+ * An attempt to delete a ref by pushing into a repository whose HEAD
+   symbolic reference points at an unborn branch that cannot be
+   created due to ref D/F conflict (e.g. refs/heads/a/b exists, HEAD
+   points at refs/heads/a) failed.
+
+ * "git subtree" (in contrib/) depended on "git log" output to be
+   stable, which was a no-no.  Apply a workaround to force a
+   particular date format.
+
+Also contains typofixes, documentation updates and trivial code
+clean-ups.