Add a new function, resolve_ref_recursively(), which is basically like
the old resolve_ref_unsafe() except that it takes a (ref_store *)
argument and also works for submodules.
Re-implement resolve_ref_unsafe() as a thin wrapper around
resolve_ref_recursively().
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>