CommitService.ListCommitsByOid: initial implementation

It actually fixed the Commit page right away.
Direct links in Notes work as well (as expected).

Error treatment is coarse: the point is to have a proper
NOT_FOUND rather than a crash. Perhaps we will have eventually
to ignore problematic oids instead, we'll see that later.

Closes #22
4 jobs for topic/default/batch_by_oids in 1 minute and 35 seconds