Adding `git ls-files` scanner

Create a new scanner based on the command `git ls-files`.

The command `git ls-files` can be used to generate a list of files that are stored in the current git repository. For very large repositories, this can be much faster than scanning the directory tree. Furthermore, this removes untracked files (such as binary files) from the Command-T file listing.

The scanner is enabled by setting the parameter `g:CommandTUseGitLsFiles`.