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)

modules/git/repo_test.go

@@ -15,7 +15,7 @@ import (
 
 func TestRepoIsEmpty(t *testing.T) {
 	emptyRepo2Path := filepath.Join(testReposDir, "repo2_empty")
-	repo, err := OpenRepositoryLocal(emptyRepo2Path)
+	repo, err := OpenRepositoryLocal(t.Context(), emptyRepo2Path)
 	assert.NoError(t, err)
 	defer repo.Close()
 	isEmpty, err := repo.IsEmpty(t.Context())