core: teach `git cola` about `gui.displayuntracked`

Make the data gathering step honor the `gui.displayuntracked`
variable.  When set false, `git cola` will not display untracked
files in the files list.  Teach the preferences dialog to toggle
this setting.

C.f. http://thread.gmane.org/gmane.comp.version-control.git/232683
Signed-off-by: David Aguilar <davvid@gmail.com>