rebase -r: always reword merge -c

If a merge can be fast-forwarded then make sure that we still edit the
commit message if the user specifies -c. The implementation follows the
same pattern that is used for ordinary rewords that are fast-forwarded.
Signed-off-by: NPhillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

sequencer.c

@@ -3401,6 +3401,10 @@ static int do_merge(struct repository *r,
 		rollback_lock_file(&lock);
 		ret = fast_forward_to(r, &commit->object.oid,
 				      &head_commit->object.oid, 0, opts);
+		if (flags & TODO_EDIT_MERGE_MSG) {
+			run_commit_flags |= AMEND_MSG;
+			goto fast_forward_edit;
+		}
 		goto leave_merge;
 	}
 
@@ -3504,6 +3508,7 @@ static int do_merge(struct repository *r,
 		 * value (a negative one would indicate that the `merge`
 		 * command needs to be rescheduled).
 		 */
+	fast_forward_edit:
 		ret = !!run_git_commit(r, git_path_merge_msg(r), opts,
 				       run_commit_flags);
 

t/t3430-rebase-merges.sh

@@ -164,6 +164,19 @@ test_expect_success 'failed `merge <branch>` does not crash' '
 	grep "^Merge branch ${SQ}G${SQ}$" .git/rebase-merge/message
 '
 
+test_expect_success 'fast-forward merge -c still rewords' '
+	git checkout -b fast-forward-merge-c H &&
+	(
+		set_fake_editor &&
+		FAKE_COMMIT_MESSAGE=edited \
+			GIT_SEQUENCE_EDITOR="echo merge -c H G >" \
+			git rebase -ir @^
+	) &&
+	echo edited >expected &&
+	git log --pretty=format:%B -1 >actual &&
+	test_cmp expected actual
+'
+
 test_expect_success 'with a branch tip that was cherry-picked already' '
 	git checkout -b already-upstream master &&
 	base="$(git rev-parse --verify HEAD)" &&