fetch: convert strncmp() with strlen() to starts_with()

Using strncmp() and strlen() to check whether a string starts with
another one requires repeating the prefix candidate.  Use starts_with()
instead, which reduces repetition and is more readable.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: rscharfe

builtin/fetch.c

@@ -448,9 +448,8 @@ static void filter_prefetch_refspec(struct refspec *rs)
 			continue;
 		if (!rs->items[i].dst ||
 		    (rs->items[i].src &&
-		     !strncmp(rs->items[i].src,
-			      ref_namespace[NAMESPACE_TAGS].ref,
-			      strlen(ref_namespace[NAMESPACE_TAGS].ref)))) {
+		     starts_with(rs->items[i].src,
+				 ref_namespace[NAMESPACE_TAGS].ref))) {
 			int j;
 
 			free(rs->items[i].src);