Allow push mirrors to disable the "delete diverged refs" behavior

This will resolve limitation number 1 from gitlab#198270:

Push mirroring will delete anything in the destination that isn't in the source. This means we can't create the auto-deploy branch solely on Security, as Canonical's push mirror will attempt to delete it. If the branch is protected the mirroring will be blocked entirely; if it's unprotected it gets deleted.

Proposal

This will require an additional field in the UpdateRemoteMirrorRequest protocol. Something like keep_diverged_refs, which defaults to false since it's the current behavior.

When enabled, we won't perform the delete_refs calls for branches or tags.

This doesn't need to be behind a Gitaly Ruby feature flag, because it will be part of the protocol and default to the current behavior. The gitlab-rails side to allow enabling the functionality will be behind a feature flag.