CommitService: missing resolutions of obsolete changesets

The Rails app will legitimately call a lot of methods on obsolete changesets, for instance to watch older commits in MRs, or to maintain direct commit links. It is a general principle that this must work if the commit is identified by its full id (hash), but there were some loopholes and it was poorly tested.

Closes heptapod#640 (closed), which is due to ListCommitByOids