ref-cache: use a callback function to fill the cache

ref-cache: use a callback function to fill the cache

It is a leveling violation for `ref_cache` to know about
`files_ref_store` or that it should call `read_loose_refs()` to lazily
fill cache directories. So instead, have its constructor take as an
argument a callback function that it should use for lazy-filling, and
change `files_ref_store` to supply a pointer to function
`read_loose_refs` (renamed to `loose_fill_ref_dir`) when creating the
ref cache for its loose refs.

This means that we can generify the type of the back-pointer in
`struct ref_cache` from `files_ref_store` to `ref_store`.

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