refs: support obtaining ref_store for given dir

refs: support obtaining ref_store for given dir

The refs subsystem uses the `get_main_ref_store()` to obtain the main
ref_store for a given repository. In the upcoming patches we also want
to create a ref_store for any given reference directory, which may exist
in arbitrary paths. For the files backend and the reftable backend, the
reference directory is generally the $GIT_DIR.

To support such behavior, extract out the core logic for creating out
the ref_store from `get_main_ref_store()` into a new function
`get_ref_store_for_dir()` which can provide the ref_store for a
given (repository, directory, reference format) combination.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>