+ if (!memcmp(ref->old_sha1, ref->peer_ref->new_sha1, 20)) {
+ if (verbose)
+ fprintf(stderr, "'%s': up-to-date\n", ref->name);
+ continue;
+ }
+
+ /* This part determines what can overwrite what.
+ * The rules are:
+ *
+ * (0) you can always use --force or +A:B notation to
+ * selectively force individual ref pairs.
+ *
+ * (1) if the old thing does not exist, it is OK.
+ *
+ * (2) if you do not have the old thing, you are not allowed
+ * to overwrite it; you would not know what you are losing
+ * otherwise.
+ *
+ * (3) if both new and old are commit-ish, and new is a
+ * descendant of old, it is OK.
+ */
+
+ if (!force_update &&
+ !is_zero_sha1(ref->old_sha1) &&
+ !ref->force) {