Advertising history to support finding a common merge base isn't
entirely part of ReceiveCommits. Its just a feature hacked into the
code to support Gerrit Code Review's unique workflow where clients
are frequently behind the server.
Pull this code into its own hook, fired after the ReceiveCommits
specific hook that hides refs/changes and shows open changes as
additional objects.
This refactoring is mostly selfish; the server environment at Google
can benefit from these two unrelated concepts being separate hooks.
Change-Id: I91536f79d7605daf6136eaf8c5417729bfe56c15
1ca0129613