Fixed issue #1773: "Show changes as unified diff" on "Working dir changes" show inverted diff operation

Regression of revision 1c5766fe494a46547b5baaccda5473857a155894.

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Update Tortoise.pot

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>