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)

models/git/commit_status_test.go

@@ -186,7 +186,7 @@ func TestFindRepoRecentCommitStatusContexts(t *testing.T) {
 
 	repo2 := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: 2})
 	user2 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 2})
-	gitRepo, err := git.OpenRepository(repo2)
+	gitRepo, err := git.OpenRepository(t.Context(), repo2)
 	assert.NoError(t, err)
 	defer gitRepo.Close()