Fix nested submodules

Make the `Repository#submodules` method work for submodules that are
nested below the root repo directory.
Adapt the tests to the latest gitlab-git-test repo
Add tests for nested submodules
Merge branch 'adapt_test_to_new_repo' into 'master'

Adapt the tests to the latest gitlab-git-test repo

See merge request !10
Merge branch 'nested-submodules' into 'master'

Fix nested submodules

Make the `Repository#submodules` method work with submodules that are nested beneath the repo's root directory.  This patch uses the existing `#tree_entry` method to recurse through the commit tree as an alternative to the patch in #6.

Fixes [issue 8135](https://github.com/gitlabhq/gitlabhq/issues/8135) on github.

/cc @jacobvosmaer @dzaporozhets

See merge request !9
Bump VERSION to 7.0.0.rc10
Merge branch 'master' of github.com:gitlabhq/gitlab_git