commit.c: use skip_prefix() instead of starts_with()

In record_author_date() & parse_gpg_output(), the callers of
starts_with() not just want to know if the string starts with the
prefix, but also can benefit from knowing the string that follows
the prefix.

By using skip_prefix(), we can do both at the same time.

Helped-by: Max Horn <max@quendi.de>
Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Tanay Abhra <tanayabh@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -548,7 +548,7 @@ define_commit_slab(author_date_slab, unsigned long);
 static void record_author_date(struct author_date_slab *author_date,
 			       struct commit *commit)
 {
-	const char *buf, *line_end;
+	const char *buf, *line_end, *ident_line;
 	char *buffer = NULL;
 	struct ident_split ident;
 	char *date_end;
@@ -566,14 +566,14 @@ static void record_author_date(struct author_date_slab *author_date,
 	     buf;
 	     buf = line_end + 1) {
 		line_end = strchrnul(buf, '\n');
-		if (!starts_with(buf, "author ")) {
+		ident_line = skip_prefix(buf, "author ");
+		if (!ident_line) {
 			if (!line_end[0] || line_end[1] == '\n')
 				return; /* end of header */
 			continue;
 		}
 		if (split_ident_line(&ident,
-				     buf + strlen("author "),
-				     line_end - (buf + strlen("author "))) ||
+				     ident_line, line_end - ident_line) ||
 		    !ident.date_begin || !ident.date_end)
 			goto fail_exit; /* malformed "author" line */
 		break;
@@ -1193,10 +1193,8 @@ static void parse_gpg_output(struct signature_check *sigc)
 	for (i = 0; i < ARRAY_SIZE(sigcheck_gpg_status); i++) {
 		const char *found, *next;
 
-		if (starts_with(buf, sigcheck_gpg_status[i].check + 1)) {
-			/* At the very beginning of the buffer */
-			found = buf + strlen(sigcheck_gpg_status[i].check + 1);
-		} else {
+		found = skip_prefix(buf, sigcheck_gpg_status[i].check + 1);
+		if (!found) {
 			found = strstr(buf, sigcheck_gpg_status[i].check);
 			if (!found)
 				continue;