trailer: forbid leading whitespace in trailers

Currently, interpret-trailers allows leading whitespace in trailer
lines. This leads to false positives, especially for quoted lines or
bullet lists.

Forbid leading whitespace in trailers.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-interpret-trailers.txt

@@ -55,7 +55,7 @@ The group must either be at the end of the message or be the last
 non-whitespace lines before a line that starts with '---'. Such three
 minus signs start the patch part of the message.
 
-When reading trailers, there can be whitespaces before and after the
+When reading trailers, there can be whitespaces after the
 token, the separator and the value. There can also be whitespaces
 inside the token and the value.
 

t/t7513-interpret-trailers.sh

@@ -241,6 +241,21 @@ test_expect_success 'with non-trailer lines only' '
 	test_cmp expected actual
 '
 
+test_expect_success 'line with leading whitespace is not trailer' '
+	q_to_tab >patch <<-\EOF &&
+
+		Qtoken: value
+	EOF
+	q_to_tab >expected <<-\EOF &&
+
+		Qtoken: value
+
+		token: value
+	EOF
+	git interpret-trailers --trailer "token: value" patch >actual &&
+	test_cmp expected actual
+'
+
 test_expect_success 'with config setup' '
 	git config trailer.ack.key "Acked-by: " &&
 	cat >expected <<-\EOF &&

trailer.c

@@ -775,7 +775,7 @@ static int find_trailer_start(struct strbuf **lines, int count)
 		}
 
 		separator_pos = find_separator(lines[start]->buf);
-		if (separator_pos >= 1) {
+		if (separator_pos >= 1 && !isspace(lines[start]->buf[0])) {
 			struct list_head *pos;
 
 			trailer_lines++;