Allow for obeying of .gitignore

[hibachrach]

I typically have to run codemod on several directories in my working repository with the -d option. I would run it in the root of my working repository, but it includes some large irrelevant files that slow codemod down. Is there a way to have it obey .gitignore?

[modocache]

This would be a good feature! Maybe codemod could use git ls-files or something to find the list of files, instead of Python's os.walk?