rebase -i: introduce --rebase-merges=[no-]rebase-cousins

When running `git rebase --rebase-merges` non-interactively with an
ancestor of HEAD as <upstream> (or leaving the todo list unmodified),
we would ideally recreate the exact same commits as before the rebase.

However, if there are commits in the commit range <upstream>.. that do not
have <upstream> as direct ancestor (i.e. if `git log <upstream>..` would
show commits that are omitted by `git log --ancestry-path <upstream>..`),
this is currently not the case: we would turn them into commits that have
<upstream> as direct ancestor.

Let's illustrate that with a diagram:

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

Currently, after running `git rebase -i --rebase-merges B`, the new branch
structure would be (pay particular attention to the commit `D`):

       --- 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 --rebase-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. For hypothetical
use cases where cousins *do* need to be rebased, `git rebase
--rebase=merges=rebase-cousins` needs to be used.

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

1 files changed, 18 insertions, 0 deletions

diff --git a/t/t3430-rebase-merges.sh b/t/t3430-rebase-merges.sh
index 1628c8dcc2..3d4dfdf7be 100755
--- a/t/t3430-rebase-merges.sh
+++ b/t/t3430-rebase-merges.sh
@@ -176,6 +176,24 @@ test_expect_success 'with a branch tip that was cherry-picked already' '
 	EOF
 '
 
+test_expect_success 'do not rebase cousins unless asked for' '
+	git checkout -b cousins master &&
+	before="$(git rev-parse --verify HEAD)" &&
+	test_tick &&
+	git rebase -r HEAD^ &&
+	test_cmp_rev HEAD $before &&
+	test_tick &&
+	git rebase --rebase-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 &&