Merge branch 'hn/history-squash' into seen

The experimental "git history" command has been taught a new
"squash" subcommand to fold a range of commits into a single commit,
replaying any descendants on top.

* hn/history-squash:
  SQUASH??? avoid test_grep lint triggering on uses of raw grep
  SQUASH??? avoid test number clash by moving 3454-h-squash to 3455
  history: re-edit a squash with every message
  history: add squash subcommand to fold a range
  history: give commit_tree_ext a message template
  history: extract helper for a commit's parent tree

Author: gitster

Documentation/git-history.adoc

@@ -12,6 +12,7 @@ git history drop <commit> [--dry-run] [--update-refs=(branches|head)] [--empty=(
 git history fixup <commit> [--dry-run] [--update-refs=(branches|head)] [--reedit-message] [--empty=(drop|keep|abort)]
 git history reword <commit> [--dry-run] [--update-refs=(branches|head)]
 git history split <commit> [--dry-run] [--update-refs=(branches|head)] [--] [<pathspec>...]
+git history squash <revision-range> [--dry-run] [--update-refs=(branches|head)] [--reedit-message]
 
 DESCRIPTION
 -----------
@@ -113,6 +114,26 @@ linkgit:gitglossary[7].
 It is invalid to select either all or no hunks, as that would lead to
 one of the commits becoming empty.
 
+`squash <revision-range>`::
+	Fold all commits in _<revision-range>_ into the oldest commit of that
+	range. The resulting commit keeps the oldest commit's message and
+	authorship and takes the tree of the range's newest commit, so the
+	whole range collapses into a single commit. Commits above the range
+	are replayed on top of the result.
++
+The range is given in the usual `<base>..<tip>` form, where _<base>_ is
+the commit just below the oldest commit to squash. For example, `git
+history squash @~3..` folds the three most recent commits into one, and
+`git history squash @~5..@~2` squashes an interior range while leaving
+the two newest commits in place.
++
+The oldest commit's message and authorship are preserved by default. With
+`--reedit-message`, an editor opens pre-filled with the messages of all the
+folded commits so you can combine them. A merge commit inside the range is
+folded like any other, but the range must have a single base, so a range
+that reaches more than one entry point (for example a side branch that
+forked before the range and was later merged into it) is rejected.
+
 OPTIONS
 -------