When processing status for a newly checked out repo, it is
possible that there will be submodules that have not yet been
initialized. The only way to distinguish these from untracked
directories is to have some knowledge of submodules. This
commit adds a new submodule API which, given a name or path,
can determine if it appears to be a submodule and can give
information about the submodule.
This "fixes" the broken t18 status tests to accurately reflect
the new behavior for "created" untracked subdirectories. See
discussion in the PR for more details.
This also contains the submodules unit test that I forgot to
git add, and ports most of the t18-status.c tests to clar (still
missing a couple of the git_status_file() single file tests).
