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/grep_test.go

@@ -16,7 +16,7 @@ import (
 
 func TestGrepSearch(t *testing.T) {
 	defer test.MockVariableValue(&setting.RepoRootPath, t.TempDir())()
-	repo, err := OpenRepositoryLocal(filepath.Join(testReposDir, "language_stats_repo"))
+	repo, err := OpenRepositoryLocal(t.Context(), filepath.Join(testReposDir, "language_stats_repo"))
 	assert.NoError(t, err)
 	defer repo.Close()