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_gogit.go

@@ -31,7 +31,8 @@ type Repository struct {
 	gogitStorage *filesystem.Storage
 	gpgSettings  *GPGSettings
 
-	Ctx context.Context
+	Ctx             context.Context
+	LastCommitCache *LastCommitCache
 }
 
 // openRepositoryWithDefaultContext opens the repository at the given path with DefaultContext.
@@ -79,6 +80,8 @@ func (repo *Repository) Close() (err error) {
 	if err := repo.gogitStorage.Close(); err != nil {
 		gitealog.Error("Error closing storage: %v", err)
 	}
+	repo.LastCommitCache = nil
+	repo.tagCache = nil
 	return
 }