refs: introduce get_worktree_ref_store()

files-backend at this point is still aware of the per-repo/worktree
separation in refs, so it can handle a linked worktree.

Some refs operations are known not working when current files-backend is
used in a linked worktree (e.g. reflog). Tests will be written when
refs_* functions start to be called with worktree backend to verify that
they work as expected.

Note: accessing a worktree of a submodule remains unaddressed. Perhaps
after get_worktrees() can access submodule (or rather a new function
get_submodule_worktrees(), that lists worktrees of a submodule), we can
update this function to work with submodules as well.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.h

@@ -5,6 +5,7 @@ struct object_id;
 struct ref_store;
 struct strbuf;
 struct string_list;
+struct worktree;
 
 /*
  * Resolve a reference, recursively following symbolic refererences.
@@ -655,5 +656,6 @@ struct ref_store *get_main_ref_store(void);
  * submodule==NULL.
  */
 struct ref_store *get_submodule_ref_store(const char *submodule);
+struct ref_store *get_worktree_ref_store(const struct worktree *wt);
 
 #endif /* REFS_H */