Check null pointer in CGitCall_ByteVector::OnOutputData()

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
Add flag indication user can change Read only mode for view

Based on TortoiseSVN revision 24075

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
Add the "Enable Edit mode" button to the ribbon and the File menu
* Set the button as enabled if it's possible to change the readonly state
* Set the button as checked if the current view is editable
* remove state indication from the view header

Based on TortoiseSVN revision 24076

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