Slightly simplify LastCommitCache (#20444)

The LastCommitCache code is a little complex and there is unnecessary
duplication between the gogit and nogogit variants.

This PR adds the LastCommitCache as a field to the git.Repository and
pre-creates it in the ReferencesGit helpers etc. There has been some
simplification and unification of the variant code.

Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/git/notes_gogit.go

@@ -83,7 +83,7 @@ func GetNote(ctx context.Context, repo *Repository, commitID string, note *Note)
 		log.Error("Unable to get the commit for the path %q. Error: %v", path, err)
 		return err
 	}
-	note.Commit = convertCommit(lastCommits[path])
+	note.Commit = lastCommits[path]
 
 	return nil
 }