RHGitaly implement CommitService.FindCommit

According to the statistics just extracted from foss.heptapod.net, this is by far the most frequently called method not already handled by RHGitaly:

TOTAL Requests: 679185
Breakdown:
  47.5% FindCommitRequest (count=322444)
   9.4% RefExistsRequest (count=64032)
   7.5% TreeEntryRequest (count=50906)
   6.1% FindCommitsRequest (count=41316)
   4.7% GetTreeEntriesRequest (count=32113)
   3.8% HasLocalBranchesRequest (count=25758)
   2.7% GetBlobsRequest (count=18674)

(statistics between 2023-04-29 18:47:13,342 and now)

This is a low hanging fruit after #126 (closed) allows us to translate the incoming revision into a ChangelogEntry, given that we already have the conversion of the latter into a gRPC GitCommit message.

The Python implementation is probably one of the fastest, but the volume is impressive.