Apparently nobody really makes use of git-diff-stages, as nobody
has complained that it is not supported by the git-diff frontend.
Since its likely this will go away in the future, we should not
offer it as a possible subcommand completion.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org> Signed-off-by: Junio C Hamano <junkio@cox.net>