+ /* 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) {
+ if (!has_sha1_file(ref->old_sha1)) {
+ error("remote '%s' object %s does not "
+ "exist on local",
+ ref->name, sha1_to_hex(ref->old_sha1));
+ continue;
+ }
+
+ /* We assume that local is fsck-clean. Otherwise
+ * you _could_ have an old tag which points at
+ * something you do not have, which may or may not
+ * be a commit.
+ */
+ if (!ref_newer(ref->peer_ref->new_sha1,
+ ref->old_sha1)) {
+ error("remote ref '%s' is not a strict "
+ "subset of local ref '%s'.", ref->name,
+ ref->peer_ref->name);
+ continue;
+ }
+ }
+ memcpy(ref->new_sha1, ref->peer_ref->new_sha1, 20);
+ if (is_zero_sha1(ref->new_sha1)) {
+ error("cannot happen anymore");
+ continue;
+ }
+ new_refs++;