VcsBase: Abort commands when closing VcsBaseEditorWidget

This prevents the crashes when the command signals a state change
after the managing editor was closed.

Task-number: QTCREATORBUG-10343
Change-Id: I57e34c49d5ef8b5693bec6ac0ebc9d87afa99dc9
Reviewed-by: Orgad Shaneh <orgads@gmail.com>