Add :Glsfiles, :Gllsfiles commands

These put the output of git-ls-files in the error/location list.

Note that they currently assume that the output is just the file names,
so running `:Glsfiles -s` will not work properly.

Feel free to kill this if you do not think it is a useful enough feature or to tell me to make a bunch of changes to it; you're not going to hurt my feelings. I also don't mind if you take a while to get back to me, especially since I think we agree that this is a fairly low-priority issue. 

refs #132