Add Close() method to gogitRepository (#8901)

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

docs/content/doc/advanced/migrations.en-us.md

@@ -68,6 +68,7 @@ type Uploader interface {
 	CreateComment(issueNumber int64, comment *Comment) error
 	CreatePullRequest(pr *PullRequest) error
 	Rollback() error
+	Close()
 }
 
-```
+```