refs: change signatures of get_packed_refs() and get_loose_refs()

refs: change signatures of get_packed_refs() and get_loose_refs()

Change get_packed_refs() and get_loose_refs() to take a (struct
ref_cache *) instead of the name of the submodule.

Change get_ref_dir() to take a submodule name (i.e., "" for the main
module) rather than a submodule pointer (i.e., NULL for the main
module) so that refs->name can be used as its argument.  (In a moment
this function will also be changed to take a (struct ref_cache *),
too.)

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>