mailinfo: fix in-body header continuations

An empty line should stop any pending in-body headers, and start the
actual body parsing.

This also modifies the original test for the in-body headers to actually
have a real commit body that starts with spaces, and changes the test to
check that the long line matches _exactly_, and doesn't get extra data
from the body.

Fixes:6b4b013f1884 ("mailinfo: handle in-body header continuations")
Cc: Jonathan Tan <jonathantanmy@google.com>
Cc: Jeff King <peff@peff.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

mailinfo.c

@@ -674,9 +674,14 @@ static int handle_commit_msg(struct mailinfo *mi, struct strbuf *line)
 	assert(!mi->filter_stage);
 
 	if (mi->header_stage) {
-		if (!line->len || (line->len == 1 && line->buf[0] == '\n'))
+		if (!line->len || (line->len == 1 && line->buf[0] == '\n')) {
+			if (mi->inbody_header_accum.len) {
+				flush_inbody_header_accum(mi);
+				mi->header_stage = 0;
+			}
 			return 0;
 		}
+	}
 
 	if (mi->use_inbody_headers && mi->header_stage) {
 		mi->header_stage = check_inbody_header(mi, line);

t/t4150-am.sh

@@ -983,7 +983,9 @@ test_expect_success 'am works with multi-line in-body headers' '
 	rm -fr .git/rebase-apply &&
 	git checkout -f first &&
 	echo one >> file &&
-	git commit -am "$LONG" --author="$LONG <long@example.com>" &&
+	git commit -am "$LONG
+
+    Body test" --author="$LONG <long@example.com>" &&
 	git format-patch --stdout -1 >patch &&
 	# bump from, date, and subject down to in-body header
 	perl -lpe "
@@ -997,7 +999,7 @@ test_expect_success 'am works with multi-line in-body headers' '
 	git am msg &&
 	# Ensure that the author and full message are present
 	git cat-file commit HEAD | grep "^author.*long@example.com" &&
-	git cat-file commit HEAD | grep "^$LONG"
+	git cat-file commit HEAD | grep "^$LONG$"
 '
 
 test_done