sequencer: ignore "---" divider when parsing trailers

When the sequencer code appends a signoff or cherry-pick
origin, it uses the default trailer-parsing options, which
treat "---" as the end of the commit message. As a result,
it may be fooled by a commit message that contains that
string and fail to find the existing trailer block. Even
more confusing, the actual append code does not know about
"---", and always appends to the end of the string. This can
lead to bizarre results. E.g., appending a signoff to a
commit message like this:

  subject

  body
  ---
  these dashes confuse the parser!

  Signed-off-by: A

results in output with a final block like:

  Signed-off-by: A

  Signed-off-by: A

The parser thinks the final line of the message is "body",
and ignores everything else, claiming there are no trailers.
So we output an extra newline separator (wrong) and add a
duplicate signoff (also wrong).

Since we know we are feeding a pure commit message, we can
simply tell the parser to ignore the "---" divider.

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

sequencer.c

@@ -229,6 +229,8 @@ static int has_conforming_footer(struct strbuf *sb, struct strbuf *sob,
 	size_t i;
 	int found_sob = 0, found_sob_last = 0;
 
+	opts.no_divider = 1;
+
 	trailer_info_get(&info, sb->buf, &opts);
 
 	if (info.trailer_start == info.trailer_end)

t/t7501-commit.sh

@@ -517,6 +517,22 @@ Myfooter: x" &&
 	test_cmp expected actual
 '
 
+test_expect_success 'signoff not confused by ---' '
+	cat >expected <<-EOF &&
+		subject
+
+		body
+		---
+		these dashes confuse the parser!
+
+		Signed-off-by: $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL>
+	EOF
+	# should be a noop, since we already signed
+	git commit --allow-empty --signoff -F expected &&
+	git log -1 --pretty=format:%B >actual &&
+	test_cmp expected actual
+'
+
 test_expect_success 'multiple -m' '
 
 	>negative &&