sequencer: handle post-rewrite for merge commands

In the previous patches, we implemented the basic functionality of the
`git rebase -i --rebase-merges` command, in particular the `merge`
command to create merge commits in the sequencer.

The interactive rebase is a lot more these days, though, than a simple
cherry-pick in a loop. For example, it calls the post-rewrite hook (if
any) after rebasing with a mapping of the old->new commits.

This patch implements the post-rewrite handling for the `merge` command
we just introduced. The other commands that were added recently (`label`
and `reset`) do not create new commits, therefore post-rewrite hooks do
not need to handle them.

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

sequencer.c

@@ -3074,7 +3074,10 @@ static int pick_commits(struct todo_list *todo_list, struct replay_opts *opts)
 					    item->arg, item->arg_len,
 					    item->flags, opts)) < 0)
 				reschedule = 1;
-			else if (res > 0)
+			else if (item->commit)
+				record_in_rewritten(&item->commit->object.oid,
+						    peek_command(todo_list, 1));
+			if (res > 0)
 				/* failed with merge conflicts */
 				return error_with_patch(item->commit,
 							item->arg,

t/t3430-rebase-merges.sh

@@ -190,4 +190,29 @@ test_expect_success 'refs/rewritten/* is worktree-local' '
 	test_cmp_rev HEAD "$(cat wt/b)"
 '
 
+test_expect_success 'post-rewrite hook and fixups work for merges' '
+	git checkout -b post-rewrite &&
+	test_commit same1 &&
+	git reset --hard HEAD^ &&
+	test_commit same2 &&
+	git merge -m "to fix up" same1 &&
+	echo same old same old >same2.t &&
+	test_tick &&
+	git commit --fixup HEAD same2.t &&
+	fixup="$(git rev-parse HEAD)" &&
+
+	mkdir -p .git/hooks &&
+	test_when_finished "rm .git/hooks/post-rewrite" &&
+	echo "cat >actual" | write_script .git/hooks/post-rewrite &&
+
+	test_tick &&
+	git rebase -i --autosquash -r HEAD^^^ &&
+	printf "%s %s\n%s %s\n%s %s\n%s %s\n" >expect $(git rev-parse \
+		$fixup^^2 HEAD^2 \
+		$fixup^^ HEAD^ \
+		$fixup^ HEAD \
+		$fixup HEAD) &&
+	test_cmp expect actual
+'
+
 test_done