rebase -i: introduce --recreate-merges=[no-]rebase-cousinsjs/rebase-recreate-merge

This one is a bit tricky to explain, so let's try with a diagram:

        C
      /   \
A - B - E - F
  \   /
    D

To illustrate what this new mode is all about, let's consider what
happens upon `git rebase -i --recreate-merges B`, in particular to
the commit `D`. So far, the new branch structure would be:

       --- C' --
      /         \
A - B ------ E' - F'
      \    /
        D'

This is not really preserving the branch topology from before! The
reason is that the commit `D` does not have `B` as ancestor, and
therefore it gets rebased onto `B`.

This is unintuitive behavior. Even worse, when recreating branch
structure, most use cases would appear to want cousins *not* to be
rebased onto the new base commit. For example, Git for Windows (the
heaviest user of the Git garden shears, which served as the blueprint
for --recreate-merges) frequently merges branches from `next` early, and
these branches certainly do *not* want to be rebased. In the example
above, the desired outcome would look like this:

       --- C' --
      /         \
A - B ------ E' - F'
  \        /
   -- D' --

Let's introduce the term "cousins" for such commits ("D" in the
example), and let's not rebase them by default, introducing the new
"rebase-cousins" mode for use cases where they should be rebased.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 23 insertions, 0 deletions

diff --git a/t/t3430-rebase-recreate-merges.sh b/t/t3430-rebase-recreate-merges.sh
index 2fb74cbbd3..f2f44d2a66 100755
--- a/t/t3430-rebase-recreate-merges.sh
+++ b/t/t3430-rebase-recreate-merges.sh
@@ -143,6 +143,29 @@ test_expect_success 'with a branch tip that was cherry-picked already' '
 	EOF
 '
 
+test_expect_success 'do not rebase cousins unless asked for' '
+	write_script copy-editor.sh <<-\EOF &&
+	cp "$1" "$(git rev-parse --git-path ORIGINAL-TODO)"
+	EOF
+
+	test_config sequence.editor \""$PWD"/copy-editor.sh\" &&
+	git checkout -b cousins master &&
+	before="$(git rev-parse --verify HEAD)" &&
+	test_tick &&
+	git rebase -i --recreate-merges HEAD^ &&
+	test_cmp_rev HEAD $before &&
+	test_tick &&
+	git rebase -i --recreate-merges=rebase-cousins HEAD^ &&
+	test_cmp_graph HEAD^.. <<-\EOF
+	*   Merge the topic branch '\''onebranch'\''
+	|\
+	| * D
+	| * G
+	|/
+	o H
+	EOF
+'
+
 test_expect_success 'refs/rewritten/* is worktree-local' '
 	git worktree add wt &&
 	cat >wt/script-from-scratch <<-\EOF &&