commit: pass --no-divider to interpret-trailers

When git-commit sees any "--trailer" options, it passes the
COMMIT_EDITMSG file through git-interpret-trailers. But it does so
without passing --no-divider, which means that interpret-trailers will
look for a "---" divider to signal the end of the commit message.

That behavior doesn't make any sense in this context; we know we have a
complete and solitary commit message, not something we have to further
parse. And as a result, we'll do the wrong thing if the commit message
contains a "---" marker (which otherwise is not syntactically
significant), inserting any new trailers at the wrong spot.

We can fix this by passing --no-divider. This is the exact situation for
which it was added in 1688c9a489 (interpret-trailers: allow suppressing
"---" divider, 2018-08-22). As noted in the message for that commit, it
just adds the mechanism, and further patches were needed to trigger it
from various callers.  We did that back then in a few spots, like
ffce7f590f (sequencer: ignore "---" divider when parsing trailers,
2018-08-22), but obviously missed this one.

Reported-by: <eric.frederich@siemens.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/commit.c

@@ -1043,7 +1043,8 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 		struct child_process run_trailer = CHILD_PROCESS_INIT;
 
 		strvec_pushl(&run_trailer.args, "interpret-trailers",
-			     "--in-place", git_path_commit_editmsg(), NULL);
+			     "--in-place", "--no-divider",
+			     git_path_commit_editmsg(), NULL);
 		strvec_pushv(&run_trailer.args, trailer_args.v);
 		run_trailer.git_cmd = 1;
 		if (run_command(&run_trailer))

t/t7502-commit-porcelain.sh

@@ -466,6 +466,25 @@ test_expect_success 'commit --trailer with -c and command' '
 	test_cmp expected actual
 '
 
+test_expect_success 'commit --trailer not confused by --- separator' '
+	cat >msg <<-\EOF &&
+	subject
+
+	body with dashes
+	---
+	in it
+	EOF
+	git commit --allow-empty --trailer="my-trailer: value" -F msg &&
+	{
+		cat msg &&
+		echo &&
+		echo "my-trailer: value"
+	} >expected &&
+	git cat-file commit HEAD >commit.msg &&
+	sed -e "1,/^\$/d" commit.msg >actual &&
+	test_cmp expected actual
+'
+
 test_expect_success 'multiple -m' '
 
 	>negative &&