Fixes ctrlp_user_command for git repositories

The patch I submitted in #245 was incorrect it was missing the changes 
from https://github.com/kien/ctrlp.vim/issues/174 which ensure that 
untracked files will also be included in the search. This fixes that.
Merge pull request #294 from lucisferre/patch-1

Fixes ctrlp_user_command for git repositories