push: clarify rejection of update to non-commit-ish

Pushes must already (by default) update to a commit-ish due to the fast-
forward check in set_ref_status_for_push().  But rejecting for not being
a fast-forward suggests the situation can be resolved with a merge.
Flag these updates (i.e., to a blob or a tree) as not forwardable so the
user is presented with more appropriate advice.

While updating *from* a tag object is potentially destructive, updating
*to* a tag is not.  Additionally, a push to the refs/tags/ hierarchy is
already excluded from fast-forwarding, and refs/heads/ is protected from
anything but commit objects by a check in write_ref_sha1().  Thus
someone fast-forwarding to a tag is probably not doing so by accident.
Since updating to a tag is benign and unlikely to cause confusion, allow
it in case someone finds the behavior useful.

Signed-off-by: Chris Rorvick <chris@rorvick.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

remote.c

@@ -1291,6 +1291,11 @@ static inline int is_forwardable(struct ref* ref)
 	if (!o || o->type != OBJ_COMMIT)
 		return 0;
 
+	/* new object must be commit-ish */
+	o = deref_tag(parse_object(ref->new_sha1), NULL, 0);
+	if (!o || o->type != OBJ_COMMIT)
+		return 0;
+
 	return 1;
 }