Follow-up from "Use paginated tree for the last commit query"

The following discussion from !91890 (merged) should be addressed:

If we don't need the tree at all, then we can expose LastCommit as a repository field. It will require only commit_sha and path to function. That will allow us to cut the tree query to Gitaly completely and request only the last commit information. It can be done as follow-up issue. WDYT?