Fix 'yotta version' in Windows

Oddly enough, this patch fixes 'yotta version' in Windows. Apparently,
git gets confused under Windows because --git-dir is specified with
back slashes and then it tries to append '.config' to that dir using
a forward slash. It seems that git doesn't have a problem inferring
the .git directory from the --work-dir argument, which is also implied
by the documentation of git-config (see core.worktree). Unable to
check patch on anything else than Windows, as my Python installation
in Linux just broke.