conversation: use certificates from repository during validation

If a new device is doing some actions for the first time, the merge
will detect a new certificate that is not yet in the certificate
store (unless if the sender is the new device).
This will block the validation. So, instead, use the certificate
in the processed commit.

GitLab: #892
Change-Id: I0ab492e700a2aa7ef4258e2bf711117f67e29fae