add tests for .gitattributes custom highlighting

This depends on being in master because rugged's .gitattributes implementation requires the .gitattributes file to be on HEAD. But @stanhu mentioned that there are tests that will break if we add stuff to master in this repo.

I'll merge this to master. Once we bump test_env.rb to this commit, tests will fail. We should fix them. :)

Status changed to merged

mentioned in commit 281d3a76