chore: still keep ctx in git.Repository struct for cat-file batch command (#38684)

Unfortunately, we can't completely remove the ctx from git.Repository,
because the CatFileBatch still heavily depends on a parent context.

If we remove the Repository ctx, the CatFileBatch will become a mess and
create a lot of unnecessary git processes.

http://localhost:3000/-/admin/monitor/perftrace

* Before: open a repo home, dozens of git processes (duplicate cat-file)
* After: only a few (no duplicate cat-file)

routers/web/repo/wiki_test.go

@@ -28,7 +28,7 @@ const (
 )
 
 func wikiEntry(t *testing.T, repo *repo_model.Repository, wikiName wiki_service.WebPath) (*git.Repository, *git.TreeEntry) {
-	wikiRepo, err := git.OpenRepository(repo.WikiStorageRepo())
+	wikiRepo, err := git.OpenRepository(t.Context(), repo.WikiStorageRepo())
 	assert.NoError(t, err)
 	t.Cleanup(func() {
 		defer wikiRepo.Close()