ref-filter: hoist signature parsing

When we parse a signature in the ref-filter code, we continually
increment the buffer pointer.  Hoist the signature parsing above the
blank line delimiting headers and body so we can find the signature when
using a header to sign the buffer.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: bk2204

ref-filter.c

@@ -1221,6 +1221,8 @@ static void find_subpos(const char *buf,
 	const char *end = buf + strlen(buf);
 	const char *sigstart;
 
+	/* parse signature first; we might not even have a subject line */
+	parse_signature(buf, end - buf, &payload, &signature);
 
 	/* skip past header until we hit empty line */
 	while (*buf && *buf != '\n') {
@@ -1232,9 +1234,6 @@ static void find_subpos(const char *buf,
 	/* skip any empty lines */
 	while (*buf == '\n')
 		buf++;
-
-	/* parse signature first; we might not even have a subject line */
-	parse_signature(buf, end - buf, &payload, &signature);
 	*sig = strbuf_detach(&signature, siglen);
 	sigstart = buf + parse_signed_buffer(buf, strlen(buf));
 
@@ -1330,7 +1329,7 @@ static void grab_sub_body_contents(struct atom_value *val, int deref, void *buf)
 			v->s = xmemdupz(sigpos, siglen);
 		else if (atom->u.contents.option == C_LINES) {
 			struct strbuf s = STRBUF_INIT;
-			const char *contents_end = bodylen + bodypos - siglen;
+			const char *contents_end = bodypos + nonsiglen;
 
 			/*  Size is the length of the message after removing the signature */
 			append_lines(&s, subpos, contents_end - subpos, atom->u.contents.nlines);