.gitattributes support

By: Piotr Kaminski

Reviewable now respects the diff settings in your .gitattributes files. For example, to disable diffs for any files in the vendor directory and use PHP higlighting for all .phpt files, you could put a .gitattributes file like this in the root of your repo:

/vendor/** -diff
*.phpt diff=php

For details on the file's syntax please refer to the git docs on the topic.