CommitService: implement ListCommitsByOid

This is the one able to lookup commits by ID (SHA) prefix, and ultimately the one behind generation of links to commits in Issue and MR comments. It does not seem to have anything to do with Git Tree and Blob oids.

In other words, this is also where the Rust persistent nodemap would shine.

The question would be the expected behaviour on ambiguous prefixes, i.e., those matching more than one changeset