Fix Exclude bug

[dkniffin]

Before, if you did something like:

Documentation:
  Exclude:
    - 'subdir/**/*.rb'

It wouldn't actually exclude that. It was because rubocop looks for those exclusions using an absolute path for the file name.

[dkniffin]

I'll look into this more, but it has to do with the filename being wrong in the git show commands in lib/rubocop/git/commit.rb, I think.