Fixed issue #1658: Checkbox 'add "cherry picked from"' has wrong anchor

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Clear %GIT_DIR% on process start to prevent from interfering gitdll and  git command line

(Fixes issue #1657)

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