Selecting items in FormCommit improved, should update diff view only when needed
restore last selected items on reload

(cherry picked from commit 28ecafa4e995d195b7784f93078d13e6868b2e83)

Conflicts:
	GitUI/CommandsDialogs/FormCommit.cs
Update ref to _currentFilesList after unstage all
Keep selection on last staged/unstaged file
Fixed selection when staging/unstaging selected lines.