Add Close() method to gogitRepository (#8901) (#8958)

Backport #8901 - Adjusted slightly for 1.9

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

integrations/api_repo_file_create_test.go

@@ -139,6 +139,7 @@ func TestAPICreateFile(t *testing.T) {
 			assert.EqualValues(t, expectedFileResponse.Commit.HTMLURL, fileResponse.Commit.HTMLURL)
 			assert.EqualValues(t, expectedFileResponse.Commit.Author.Email, fileResponse.Commit.Author.Email)
 			assert.EqualValues(t, expectedFileResponse.Commit.Author.Name, fileResponse.Commit.Author.Name)
+			gitRepo.Close()
 		}
 
 		// Test creating a file in a new branch