[FIX] Prevent execution of Created FileActivities on equal content

Incoming FileActivities are no longer executed when the local file
already has the same content. This prevents "ping pong" scenarios caused
by automatic changes to files in the background.

Instead of dropping these Activities on the recipient side, one could
prevent the creation on the sender side. But since Saros gets informed
*after* the change took place, it would be cumbersome to detect actual
content changes there.

Change-Id: I5384bcc35b09eaf587813b3b5a5bcc39c1bb26e0
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/857
Tested-by: Jenkins CI
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>