Update libgit2 again

This fixes problems with junctions

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Initialize the context menu manager: it's required for the system menu

Based on TortoiseSVN revision 24449

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
Only validate the update rectangle, not the whole window rectangle

Based on TortoiseSVN revision 23876

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