refs: fix ref storage format for submodule ref stores

When opening a submodule ref storage we accidentally use the ref storage
format of the owning repository, not of the submodule repository. As
submodules may have a different storage format than their parent repo
this can lead to bugs when trying to access the submodule ref storage
from the parent repository.

One such bug was reported when performing a recursive pull with mixed
ref stores, which fails with:

    $ git pull --recursive
    fatal: Unable to find current revision in submodule path 'path/to/sub'

The same issue occurs when adding a repository contained in the working
tree with a different ref storage format via `git submodule add`.

Fix the bug by using the submodule repository's ref storage format
instead and add some tests. Note that the test for `git submodule
status` was included as a precaution, only. The command worked alright
even without the bugfix.

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

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