is_racy_timestamp(): do not check timestamp for gitlinks

Because we do not even check the timestamp to determie if a gitlink
is up to date or not, triggering the racy-timestamp check for gitlinks
does not make sense.

This fixes the recently added test in t7506.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

read-cache.c

@@ -257,7 +257,8 @@ static int ce_match_stat_basic(struct cache_entry *ce, struct stat *st)
 
 static int is_racy_timestamp(const struct index_state *istate, struct cache_entry *ce)
 {
-	return (istate->timestamp &&
+	return (!S_ISGITLINK(ce->ce_mode) &&
+		istate->timestamp &&
 		((unsigned int)istate->timestamp) <= ce->ce_mtime);
 }