git-gui: Correct wording of the revert confirmation dialog.

We no longer describe updating the index as including changes, as we
now use the add notation used by core Git's command line tools.  So
its confusing to be talking about unincluded changes within the revert
dialog.  Instead we should used language like 'unadded changes'.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>