These are the aliases I have set in my .gitconfig for a similar (same?) purpose:

  [alias]
  ignore = !git update-index --assume-unchanged
  unignore = !git update-index --no-assume-unchanged
  ignored = !git ls-files -v | grep "^[[:lower:]]"

Usage: `git ignore foo.rb`, `git unignore foo.rb`; `git ignored` to list all currently ignored files.