VCS editors: Fix revision markers

Resets the revisions when setting the contents of text editor documents
in general.

Task-number: QTCREATORBUG-11356
Change-Id: I4069d629d8a612d42c7deae707300d50690e2db7
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>