tests: add submodule worktree test data
Create worktrees for submodule repositories. The worktrees are
created for the parent repository (e.g. the one containing
submodules) and for the contained child repository.
Adding new tests for new status command
This is a work in progress. This adds two new sets of tests,
the issue_592 tests from @nulltoken's pull request #601 and
some new tests for submodules. The submodule tests still have
issues where the status is not reported correctly. That needs
to be fixed before merge.
