Use --no-color for git diffs

[collin-mcgrath]

It took me a while to figure out why I was never getting results when running this gem, but I eventually tracked it down to my git config having git config color.ui set to always instead of auto. This was including ANSI color codes in the diffs and causing the parser to not match on anything.

The default setting is auto, which colors output when it’s going straight to a terminal, but omits the color-control codes when the output is redirected to a pipe or a file.
You can also set it to always to ignore the difference between terminals and pipes.

I think this will make the gem a bit more robust for anyone else that runs across the same issue.

[collin-mcgrath]

Annnd I now see that this duplicates #32.