After trying to remove the whole method in
8f69014af2902d8d53fe931268bec60f6858f160, this is a more gentle
approach to the method. :)

Prior to this change, new commit detection wasn't implemented in Gitaly,
this was done through: https://gitlab.com/gitlab-org/gitaly/merge_requests/779

As the new implemented got moved around a bit, the whole RevList class
got removed.

Part of https://gitlab.com/gitlab-org/gitaly/issues/1233