In investigating #7947 it has become clear that the storage component of go-git repositories needs closing.

This PR adds this Close function and adds the Close functions as necessary.

In TransferOwnership the ctx.Repo.GitRepo is closed if it is open to help prevent the risk of multiple open files.

Fixes #7947

Whenever we assign a value to err, check for it being nil.

* Fix empty commits now showing in repo overview

* add test

* make fmt

* Initial implementation for git statistics in Activity tab

* Create top user by commit count endpoint

* Add UI and update src-d/go-git dependency

* Add coloring

* Fix typo

* Move git activity stats data extraction to git module

* Fix message

* Add git code stats test