refs: retrieve worktree ref stores via associated repository

refs: retrieve worktree ref stores via associated repository

Similar as with the preceding commit, the worktree ref stores are always
looked up via `the_repository`. Also, again, those ref stores are stored
in a global map.

Refactor the code so that worktrees have a pointer to their repository.
Like this, we can move the global map into `struct repository` and stop
using `the_repository`. With this change, we can now in theory look up
worktree ref stores for repositories other than `the_repository`. In
practice, the worktree code will need further changes to look up
arbitrary worktrees.

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