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>

builtin/for-each-ref.c

@@ -193,7 +193,7 @@ static int verify_format(const char *format)
 		at = parse_atom(sp + 2, ep);
 		cp = ep + 1;
 
-		if (!memcmp(used_atom[at], "color:", 6))
+		if (starts_with(used_atom[at], "color:"))
 			need_color_reset_at_eol = !!strcmp(used_atom[at], color_reset);
 	}
 	return 0;

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 {

remote.c

@@ -1194,7 +1194,7 @@ static int match_explicit(struct ref *src, struct ref *dst,
 	case 1:
 		break;
 	case 0:
-		if (!memcmp(dst_value, "refs/", 5))
+		if (starts_with(dst_value, "refs/"))
 			matched_dst = make_linked_ref(dst_value, dst_tail);
 		else if (is_null_sha1(matched_src->new_sha1))
 			error("unable to delete '%s': remote ref does not exist",