refs: introduce missing functions that accept a `struct ref_store`

refs: introduce missing functions that accept a `struct ref_store`

While most of the functions in "refs.h" have a variant that accepts a
`struct ref_store`, some don't. Callers of these functions are thus
forced to implicitly rely on `the_repository` to figure out the ref
store that is to be used.

Introduce those missing functions to address this shortcoming.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>