bash: Hide diff-stages from completion.

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>

contrib/completion/git-completion.bash

@@ -269,6 +269,7 @@ __git_commands ()
 		cvsimport)        : import;;
 		cvsserver)        : daemon;;
 		daemon)           : daemon;;
+		diff-stages)      : nobody uses it;;
 		fsck-objects)     : plumbing;;
 		fetch-pack)       : plumbing;;
 		fmt-merge-msg)    : plumbing;;