Make stgit.config use git-repo-config.

This is an initial implementation without any sort of caching.  If we
want that, we could do it with this new API.  We could even just
simply generalize the use of ConfigOption instead.

I just copypasted __run from stgit.git to avoid too much work, but
it's definitely something to cleanup.

Signed-off-by: Yann Dirson <ydirson@altern.org>