gpg-interface: extract gpg line matching helper

Let's separate the actual line-by-line parsing of signatures
from the notion of "is this a gpg signature line". That will
make it easier to do more refactoring of this loop in future
patches.

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

gpg-interface.c

@@ -101,11 +101,16 @@ void print_signature_buffer(const struct signature_check *sigc, unsigned flags)
 		fputs(output, stderr);
 }
 
+static int is_gpg_start(const char *line)
+{
+	return starts_with(line, PGP_SIGNATURE) ||
+		starts_with(line, PGP_MESSAGE);
+}
+
 size_t parse_signature(const char *buf, size_t size)
 {
 	size_t len = 0;
-	while (len < size && !starts_with(buf + len, PGP_SIGNATURE) &&
-			!starts_with(buf + len, PGP_MESSAGE)) {
+	while (len < size && !is_gpg_start(buf + len)) {
 		const char *eol = memchr(buf + len, '\n', size - len);
 		len += eol ? eol - (buf + len) + 1 : size - len;
 	}