`git theta ls-files`

Similar to git lfs we should create a command that acts like git ls-files but only shows the ones that are tracked but git-theta.

git ls-files does several fancy things when given commit refs (only showing files from that ref, files that changed between refs etc) so the easiest solution is probably to easiest to have our version call repo.git.ls_files and then parse the output, only showing the values where git_utils.is_theta_tracked(${path}, gitattributes) is true.