git-gui: fix multi selected file operation

git-gui: fix multi selected file operation

When staging a selection of files using Shift-Click to choose a range
of files then using Ctrl-T or the Stage To Commit menu item will stage
all the selected files. However if a non-sequential range is selected
using Ctrl-Click then all but the first name selected gets staged. This
commit fixes this to properly stage all selected files by explicitly
adding the path to the list before showing the diff.

Signed-off-by: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>