DiffEditor: Preserve source for state recovery

Task-number: QTCREATORBUG-10126
Change-Id: I707e7cdac84c91d52c5661718ac48e3d79fc7c91
Reviewed-by: Petar Perisin <petar.perisin@gmail.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>