Optionally keep divergent refs when mirroring

Related: #2452 (closed)

b2928d5d - Add delete_divergent_refs to UpdateRemoteMirrorRequest protocol

3ac86dca - Optionally delete divergent refs when mirroring

This updates RemoteService and RemoteMirror to take a delete_divergent_refs parameter which defaults to true (the current behavior), and allows disabling the behavior on a per-mirror basis.

The option will be provided by the client and will be behind a feature flag there.

a9999cf3 - Invert delete_divergent_refs to keep_divergent_refs

We can't have a boolean field default to true, so we were breaking current behavior. The simplest solution was to invert the option, which we've done here.

726d9446 - Add changelog for "Keep divergent refs"

6c80b3af - Fix default value for keep_divergent_refs

ee665bbf - Add test case for KeepDivergentRefs