sequencer: comment commit messages properly

The rebase todo editor has commands like `fixup -c` which affects
the commit messages of the rebased commits.[1]  For example:

    pick hash1 <msg>
    fixup hash2 <msg>
    fixup -c hash3 <msg>

This says that hash2 and hash3 should be squashed into hash1 and
that hash3’s commit message should be used for the resulting commit.
So the user is presented with an editor where the two first commit
messages are commented out and the third is not.  However this does
not work if `core.commentChar`/`core.commentString` is in use since
the comment char is hardcoded (#) in this `sequencer.c` function.
As a result the first commit message will not be commented out.

† 1: See 9e3cebd (rebase -i: add fixup [-C | -c] command,
    2021-01-29)

Signed-off-by: Phillip Wood <[email protected]>
Co-authored-by: Phillip Wood <[email protected]>
Reported-by: Taylor Blau <[email protected]>
Signed-off-by: Kristoffer Haugsbakk <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

sequencer.c

@@ -1941,10 +1941,10 @@ static int seen_squash(struct replay_ctx *ctx)
 
 static void update_comment_bufs(struct strbuf *buf1, struct strbuf *buf2, int n)
 {
-	strbuf_setlen(buf1, 2);
+	strbuf_setlen(buf1, strlen(comment_line_str) + 1);
 	strbuf_addf(buf1, _(nth_commit_msg_fmt), n);
 	strbuf_addch(buf1, '\n');
-	strbuf_setlen(buf2, 2);
+	strbuf_setlen(buf2, strlen(comment_line_str) + 1);
 	strbuf_addf(buf2, _(skip_nth_commit_msg_fmt), n);
 	strbuf_addch(buf2, '\n');
 }
@@ -1963,8 +1963,12 @@ static void update_squash_message_for_fixup(struct strbuf *msg)
 	size_t orig_msg_len;
 	int i = 1;
 
-	strbuf_addf(&buf1, "# %s\n", _(first_commit_msg_str));
-	strbuf_addf(&buf2, "# %s\n", _(skip_first_commit_msg_str));
+	strbuf_add_commented_lines(&buf1, _(first_commit_msg_str),
+				   strlen(_(first_commit_msg_str)),
+				   comment_line_str);
+	strbuf_add_commented_lines(&buf2, _(skip_first_commit_msg_str),
+				   strlen(_(skip_first_commit_msg_str)),
+				   comment_line_str);
 	s = start = orig_msg = strbuf_detach(msg, &orig_msg_len);
 	while (s) {
 		const char *next;

t/t3437-rebase-fixup-options.sh

@@ -127,6 +127,21 @@ test_expect_success 'fixup -C with conflicts gives correct message' '
 	test_cmp expected-author actual-author
 '
 
+test_expect_success 'conflicting fixup -C after fixup with custom comment string' '
+	test_config core.commentString COMMENT &&
+	test_when_finished "test_might_fail git rebase --abort" &&
+	git checkout --detach A3 &&
+	test_must_fail env FAKE_LINES="1 fixup 2 fixup_-C 4" git rebase -i A &&
+	echo resolved >A &&
+	git add A &&
+	FAKE_COMMIT_AMEND=edited git rebase --continue &&
+	test_commit_message HEAD <<-\EOF
+	A3
+
+	edited
+	EOF
+'
+
 test_expect_success 'skipping fixup -C after fixup gives correct message' '
 	test_when_finished "test_might_fail git rebase --abort" &&
 	git checkout --detach A3 &&