[MERGE][#237067] Lock/unlock the RemoteRepository when locking/unlocking a RemoteBranch

[Andrew Bennetts]

Robert Collins wrote:
> 
> Just to check something, previously, we could lock a branch+repository
> in one RPC; is that still done?

Yes.  In fact this version does less RPCs — the old code would typically (maybe
always?) do a Repository.lock_write RPC immediately after the Branch.lock_write.
This patch doesn't do that.

-Andrew.