trailer: support multiline title

We currently ignore the first line passed to `git interpret-trailers`,
when looking for the beginning of the trailers.

Unfortunately this does not work well when a commit is created with a
line break in the title, using for example the following command:

git commit -m 'place of
code: change we made'

That's why instead of ignoring only the first line, it is better to
ignore the first paragraph.

Signed-off-by: Christian Couder <christian.couder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7513-interpret-trailers.sh

@@ -112,6 +112,20 @@ test_expect_success 'with only a title in the message' '
 	test_cmp expected actual
 '
 
+test_expect_success 'with multiline title in the message' '
+	cat >expected <<-\EOF &&
+		place of
+		code: change
+
+		Reviewed-by: Peff
+		Acked-by: Johan
+	EOF
+	printf "%s\n" "place of" "code: change" |
+	git interpret-trailers --trailer "Reviewed-by: Peff" \
+		--trailer "Acked-by: Johan" >actual &&
+	test_cmp expected actual
+'
+
 test_expect_success 'with config setup' '
 	git config trailer.ack.key "Acked-by: " &&
 	cat >expected <<-\EOF &&

trailer.c

@@ -743,15 +743,22 @@ static int find_patch_start(struct strbuf **lines, int count)
  */
 static int find_trailer_start(struct strbuf **lines, int count)
 {
-	int start, only_spaces = 1;
+	int start, end_of_title, only_spaces = 1;
+
+	/* The first paragraph is the title and cannot be trailers */
+	for (start = 0; start < count; start++) {
+		if (lines[start]->buf[0] == comment_line_char)
+			continue;
+		if (contains_only_spaces(lines[start]->buf))
+			break;
+	}
+	end_of_title = start;
 
 	/*
 	 * Get the start of the trailers by looking starting from the end
 	 * for a line with only spaces before lines with one separator.
-	 * The first line must not be analyzed as the others as it
-	 * should be either the message title or a blank line.
 	 */
-	for (start = count - 1; start >= 1; start--) {
+	for (start = count - 1; start >= end_of_title; start--) {
 		if (lines[start]->buf[0] == comment_line_char)
 			continue;
 		if (contains_only_spaces(lines[start]->buf)) {