[PATCH v4 3/3] sequencer: comment commit messages properly

[kristofferhaugsbakk@fastmail.com]

From: Kristoffer Haugsbakk <code@khaugsbakk.name>

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 9e3cebd97cb (rebase -i: add fixup [-C | -c] command,
    2021-01-29)

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Co-authored-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Reported-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
---

Notes (series):
    v4:
    • Fix commit message (`)
    
      Link: https://lore.kernel.org/git/5267b9a9c8cc5cc66979117dc4c1e4d7329e2a03.1729704370.git.code@khaugsbakk.name/T/#me80519debcd013aa8c8a5e5003c58cff7281fac9
    v3:
    • Message: Explain to the best of my knowledge what is going on here in
      the message body
    
      Link: https://lore.kernel.org/git/711b59d7-e649-4031-8924-a16fb632b4d4@gmail.com/
    • Fixed wrong/subpar use of trailers
    
      Link: https://lore.kernel.org/git/711b59d7-e649-4031-8924-a16fb632b4d4@gmail.com/
    v2:
    • Phillip contributed the test and the `strbuf_setlen` changes

Notes (meta-trailers):
    Co-authored-by: Phillip Wood <phillip.wood@dunelm.org.uk>
        Link: https://lore.kernel.org/git/cfa466b8-a87d-4b5d-b330-6c660897de48@gmail.com/#t

 sequencer.c                     | 12 ++++++++----
 t/t3437-rebase-fixup-options.sh | 15 +++++++++++++++
 2 files changed, 23 insertions(+), 4 deletions(-)

diff --git a/sequencer.c b/sequencer.c
index d26299cdea2..42a6f257cbb 100644
--- a/sequencer.c
+++ b/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;
diff --git a/t/t3437-rebase-fixup-options.sh b/t/t3437-rebase-fixup-options.sh
index 7929e2e2e3a..a4b90e881e3 100755
--- a/t/t3437-rebase-fixup-options.sh
+++ b/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 &&
-- 
2.47.0