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/repo_base_nogogit.go

@@ -32,7 +32,8 @@ type Repository struct {
 	checkReader *bufio.Reader
 	checkWriter WriteCloserError
 
-	Ctx context.Context
+	Ctx             context.Context
+	LastCommitCache *LastCommitCache
 }
 
 // openRepositoryWithDefaultContext opens the repository at the given path with DefaultContext.
@@ -101,5 +102,7 @@ func (repo *Repository) Close() (err error) {
 		repo.checkReader = nil
 		repo.checkWriter = nil
 	}
+	repo.LastCommitCache = nil
+	repo.tagCache = nil
 	return err
 }