CommitService: new methods for Mercurial specific changeset metadata
It would be a mistake to just add, e.g, branch and topic fields to the existing
GitCommit message, hence to the responses of existing calls, because it would conflict with whatever upstream could add in the future.
Hence we should have either a separate method for Mercurial specific changeset information (branch, topic, phase etc.) or an aggregated one giving everything in one
HgCommit message. The former would be a bit cheaper if that matters (probably not much at this stage)
Like #60, this is a fork of the protocol.
On the Rails side, this would allow to replace the
hg subprocess call of
HgGitRepository#hg_changeset_branch_topic by a HGitaly call.