files-backend: implement 'optimize' action

files-backend: implement 'optimize' action

With the generic `refs_optimize()` API now in place, provide the first
implementation for the 'files' reference backend. This makes the new API
functional for existing repositories and serves as the foundation for
migrating user-facing commands to the new architecture.

The implementation simply calls the existing `files_pack_refs()`
function, as 'packing' is the method used to optimize the files-based
reference store.

Wire up the new `files_optimize()` function to the `optimize` slot in
the files backend's virtual table.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: shejialuo <shejialuo@gmail.com>
Signed-off-by: Meet Soni <meetsoni3017@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>