refs: pass repo when retrieving submodule ref store

refs: pass repo when retrieving submodule ref store

Looking up submodule ref stores has two deficiencies:

  - The initialized subrepo will be attributed to `the_repository`.

  - The submodule ref store will be tracked in a global map.

This makes it impossible to have submodule ref stores for a repository
other than `the_repository`.

Modify the function to accept the parent repository as parameter and
move the global map into `struct repository`. Like this it becomes
possible to look up submodule ref stores for arbitrary repositories.

Note that this also adds a new reference to `the_repository` in
`resolve_gitlink_ref()`, which is part of the refs interfaces. This will
get adjusted in the next patch.

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