swarm: fix merge commit's validation after migration

Merge's commits were'nt checked, while comitting and while pulling.
This means that the author could be incorrect. Add a check before
doing a merge commit and when validating it.
This avoid incorrect repository in case a merge commit is generated
after a migration.

Misc: fix incorrect test

GitLab: #738

Change-Id: Id9748eb7d3dd5db023dd2ab4a30261c31af5d514