git-gui: fix exception when trying to stage with empty file list

If there is nothing to stage, there is nothing to stage. Let's not try
to, even if the file list contains nothing at all.

This fixes https://github.com/git-for-windows/git/issues/1075

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
1 file changed