[RFC] convert shortlog to use parse_options

I guess I should have searched the list _before_ creating these patches
since I just now stumbled upon some of the questions about how this
should be done for example:

http://kerneltrap.org/mailarchive/git/2008/3/1/1035344

From my testing this seems to work fine, but I may have missed a use

case.  I actually created these patches because I was annoyed that:

git shortlog --author=bohrer -s HEAD

didn't work, and this also fixes that issue.

--
Shawn