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)

services/markup/renderhelper_codepreview.go

@@ -50,7 +50,7 @@ func renderRepoFileCodePreview(ctx context.Context, opts markup.RenderCodePrevie
 		return "", util.ErrPermissionDenied
 	}
 
-	gitRepo, err := git.OpenRepository(dbRepo)
+	gitRepo, err := git.OpenRepository(ctx, dbRepo)
 	if err != nil {
 		return "", err
 	}