Phabricator does this too, but it works even if you force-push to the same branch - it lets you compare the base commit against all subsequent (force-)pushed commits onto the same ref.
Cool. Gerrit requires that each commit message have a "Change-Id: ...." footer so that it can pair up the new incoming commits with the old ones. And you have to push into its magic ref-spec namespace. Neat that Phabricator works by force-pushing over the existing branch, and just keeps track of its previous SHA1s, sorta like a server-side ref-log, exposed via its UI.
