Use starts_with() for C strings instead of memcmp()

Convert three cases of checking for a constant prefix using memcmp() to
starts_with().  This way there is no need for magic string length
constants and we avoid running over the end of the string should it be
shorter than the prefix.

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

fetch-pack.c

@@ -507,7 +507,7 @@ static void filter_refs(struct fetch_pack_args *args,
 		int keep = 0;
 		next = ref->next;
 
-		if (!memcmp(ref->name, "refs/", 5) &&
+		if (starts_with(ref->name, "refs/") &&
 		    check_refname_format(ref->name, 0))
 			; /* trash */
 		else {