upload-pack: only accept commits from "shallow" line

We only allow cuts at commits, not arbitrary objects. upload-pack will
fail eventually in register_shallow if a non-commit is given with a
generic error "Object %s is a %s, not a commit". Check it early and
give a more accurate error.

This should never show up in an ordinary session. It's for buggy
clients, or when the user manually edits .git/shallow.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

upload-pack.c

@@ -603,6 +603,8 @@ static void receive_needs(void)
 			object = parse_object(sha1);
 			if (!object)
 				die("did not find object for %s", line);
+			if (object->type != OBJ_COMMIT)
+				die("invalid shallow object %s", sha1_to_hex(sha1));
 			object->flags |= CLIENT_SHALLOW;
 			add_object_array(object, NULL, &shallows);
 			continue;