sequencer: comment --reference subject line properly

Comment the subject line used in `git cherry-pick --reference`
properly.

Follow the existing pattern and use the case described in the original
commit message[1] as the `core.commentChar` test case:

    If the user exits the editor without touching this line by mistake,
    what we prepare to become the first line of the body, i.e. "This
    reverts commit 8fa7f667 (do this and that, 2022-04-25)", ends up to
    be the title of the resulting commit.

† 1: 43966ab (revert: optionally refer to commit in the "reference"
    format, 2022-05-26)

Signed-off-by: Kristoffer Haugsbakk <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: LemmingAvalanche

sequencer.c

@@ -2341,8 +2341,8 @@ static int do_pick_commit(struct repository *r,
 		next = parent;
 		next_label = msg.parent_label;
 		if (opts->commit_use_reference) {
-			strbuf_addstr(&ctx->message,
-				"# *** SAY WHY WE ARE REVERTING ON THE TITLE LINE ***");
+			strbuf_commented_addf(&ctx->message, comment_line_str,
+				"*** SAY WHY WE ARE REVERTING ON THE TITLE LINE ***");
 		} else if (skip_prefix(msg.subject, "Revert \"", &orig_subject) &&
 			   /*
 			    * We don't touch pre-existing repeated reverts, because
@@ -2352,12 +2352,13 @@ static int do_pick_commit(struct repository *r,
 			   !starts_with(orig_subject, "Revert \"")) {
 			strbuf_addstr(&ctx->message, "Reapply \"");
 			strbuf_addstr(&ctx->message, orig_subject);
+			strbuf_addstr(&ctx->message, "\n");
 		} else {
 			strbuf_addstr(&ctx->message, "Revert \"");
 			strbuf_addstr(&ctx->message, msg.subject);
-			strbuf_addstr(&ctx->message, "\"");
+			strbuf_addstr(&ctx->message, "\"\n");
 		}
-		strbuf_addstr(&ctx->message, "\n\nThis reverts commit ");
+		strbuf_addstr(&ctx->message, "\nThis reverts commit ");
 		refer_to_commit(opts, &ctx->message, commit);
 
 		if (commit->parents && commit->parents->next) {

t/t3501-revert-cherry-pick.sh

@@ -228,6 +228,18 @@ test_expect_success 'identification of reverted commit (--reference)' '
 	test_cmp expect actual
 '
 
+test_expect_success 'git revert --reference with core.commentChar' '
+	test_when_finished "git reset --hard to-ident" &&
+	git checkout --detach to-ident &&
+	git -c core.commentChar=% revert \
+		--edit --reference HEAD &&
+	git log -1 --format=%B HEAD >actual &&
+	printf "This reverts commit $(git show -s \
+		--pretty=reference HEAD^).\n\n" \
+		>expect &&
+	test_cmp expect actual
+'
+
 test_expect_success 'identification of reverted commit (revert.reference)' '
 	git checkout --detach to-ident &&
 	git -c revert.reference=true revert --no-edit HEAD &&