Teach rebase -i about --preserve-merges

The option "-p" (or long "--preserve-merges") makes it possible to
rebase side branches including merges, without straightening the
history.

Example:

           X
            \
         A---M---B
        /
---o---O---P---Q

When the current HEAD is "B", "git rebase -i -p --onto Q O" will yield

               X
                 \
---o---O---P---Q---A'---M'---B'

Note that this will

- _not_ touch X [*1*], it does

- _not_ work without the --interactive flag [*2*], it does

- _not_ guess the type of the merge, but blindly uses recursive or
  whatever strategy you provided with "-s <strategy>" for all merges it
  has to redo, and it does

- _not_ make use of the original merge commit via git-rerere.

*1*: only commits which reach a merge base between <upstream> and HEAD
     are reapplied. The others are kept as-are.

*2*: git-rebase without --interactive is inherently patch based (at
     least at the moment), and therefore merges cannot be preserved.

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

1 files changed, 22 insertions, 0 deletions

diff --git a/t/t3404-rebase-interactive.sh b/t/t3404-rebase-interactive.sh
index 9f12bb9321..883cf29595 100755
--- a/t/t3404-rebase-interactive.sh
+++ b/t/t3404-rebase-interactive.sh
@@ -166,4 +166,26 @@ test_expect_success 'retain authorship when squashing' '
 	git show HEAD | grep "^Author: Nitfol"
 '
 
+test_expect_success 'preserve merges with -p' '
+	git checkout -b to-be-preserved master^ &&
+	: > unrelated-file &&
+	git add unrelated-file &&
+	test_tick &&
+	git commit -m "unrelated" &&
+	git checkout -b to-be-rebased master &&
+	echo B > file1 &&
+	test_tick &&
+	git commit -m J file1 &&
+	test_tick &&
+	git merge to-be-preserved &&
+	echo C > file1 &&
+	test_tick &&
+	git commit -m K file1 &&
+	git rebase -i -p --onto branch1 master &&
+	test $(git rev-parse HEAD^^2) = $(git rev-parse to-be-preserved) &&
+	test $(git rev-parse HEAD~3) = $(git rev-parse branch1) &&
+	test $(git show HEAD:file1) = C &&
+	test $(git show HEAD~2:file1) = B
+'
+
 test_done