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
Status Job ID Name Coverage
  Main
passed #118970
tests-current

00:00:32

100.0%
 
  Compat
passed #118971
check-sdist

00:00:28

passed #118973
tests-hg-default

00:01:02

100.0%
passed #118972
tests-hg-stable

00:00:35

100.0%