Fixed 0cad bug with smaller fix.

Fixed 0cad bug with smaller fix.

Hubert Chathi's fix was confusing for me, so I made a simpler change.
Seems to work so far.  The problem was that

os.path.dirname('filename')

returns an empty string ('') if there are no directories in the
filename.  So when `git rev-parse --git-dir` returned '.git', os
returned ''.  Later programs didn't recognize '' as a valid directory
and crashed.  My fix returns '.' in this case, so we don't crash,
and avoid having to use full paths.  I'm not sure why I don't want
to use full paths; they just give me bad vibes...