Add support for 'git findreviewers'

Summary:
It just runs findreviewers.py.

I don't know how to make 'git help findreviewers' do something useful,
but that would be cool if it could.

Test Plan:
In webapp, ran

   % git findreviewers
   % git findreviewers -r HEAD~3
   csilvers: 10 lines (100.0%)
   % git findreviewers -r HEAD~3 -f
   csilvers: 10 lines (100.0%)

   --- /home/csilvers/khan/devtools/khan-dotfiles/.gitconfig.khan

   --- /home/csilvers/khan/devtools/khan-dotfiles/mac-setup.sh
   csilvers: 8 lines (100.0%)

   --- /home/csilvers/khan/devtools/khan-dotfiles/setup.sh
   csilvers: 2 lines (100.0%)

which all seem to be as expected.

Reviewers: benkomalo, alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D2214