Let's assume mail structure with replies like this:

[1/2] Patch 1
  |- [2/2] Patch 2
  |- [2/2] Another Patch 2

When the "Another Patch 2" comes in, it's going to get inserted in the
same revision at the same place as the original "Patch 2", which results
in integrity error.

This is the single most common reason for administration notifications.

There is not really much we can do, as the reasons for such emails are
so ambiguous that the best we can do is to discard them, just like we
are doing now. So let's make it silent.

Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>