fix parsing of @{-1}@{u} combination

fix parsing of @{-1}@{u} combination

Previously interpret_branch_name would see @{-1} and stop
parsing, leaving the @{u} as cruft that provoked an error.
Instead, we should recurse if there is more to parse.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>