Add --full-name to ls-files command

[mikewadsten]

According to the git ls-files documentation,

--full-name
      When run from a subdirectory, the command usually outputs paths relative to the current directory. This option forces paths to be output relative to the project top directory.

We currently use expand or fnamemodify to generate the path to ignored fies using the relative paths returned from git ls-files with the current path prepended to it (e.g. /home/mike/git/vim-gitwildignore/plugin/../foo. By adding the --full-name argument to git ls-files we can save ourselves the trouble of doing that.