Prevent git from using color in git log

This pull request is incomplete. It fixes an issue with git log that is logged here:
https://github.com/schacon/ruby-git/issues/30
https://github.com/mroth/lolcommits/issues/50

Before proceeding i'd like to add a test, and see where else the "--no-color" argument should go. There also might be a better way to do this, by overriding config rather than passing --no-color (this would be less brittle too).