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/indexer/stats/db.go

@@ -36,7 +36,7 @@ func (db *DBIndexer) Index(id int64) error {
 		return err
 	}
 
-	gitRepo, err := git.OpenRepository(repo)
+	gitRepo, err := git.OpenRepository(ctx, repo)
 	if err != nil {
 		if err.Error() == "no such file or directory" {
 			return nil