add stage to gitignore

This is a generated builtin since 24b1f65f (Install git-stage in
exec-path).
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

.gitignore

@@ -118,6 +118,7 @@ git-show
 git-show-branch
 git-show-index
 git-show-ref
+git-stage
 git-stash
 git-status
 git-stripspace