Adding basic git ls-files support

[sherrman]

• Tracked and untracked files
• defcustom for disabling git ls-files usage
• defcustom for disabling untracked files (i.e. show only tracked files
  in the results).

[sherrman]

git ls-files doesn't support filtering directories. When I get some more time, I think that using some emacs lisp to filter out just directories would work, and I'll fix that plus the tests.

[d11wtq]

@sherrman thanks! I'll try this out later. From a quick look at git ls-files, it does seem like you'd be able to just post-process the list to filter out directories, but another solution would be to just call the original function if type is directory. Let me know if the tests cause you any grief.