Can undo mark block

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
[WIP] TMerge save marked blocks

This continues http://tortoisesvn.tigris.org/ds/viewMessage.do?dsForumId=757&dsMessageId=3074458

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