fix root updates bug in hash_builder

[flame4]

Hi, Thanks for implementing such an amazing crate to help merkle particia tree calculation.

During using, we found some weird updating case and made the smallest reproducible prototype. we add a test_updates_root test case to track updating, but nothing generated. and then we add a test_top_branch_logic to track every branch in codebase update. And we think the store_branch_node has some logic bug.

This bug will leads to that some branch nodes are not pushed into updates currently, but it doesn't affect the root calculation progress since it has no matters with the stack logic.

To reproduce the error, maybe you can just call test_updates_root directly. The origin call will get an empty updates, which is not reasonable.

We also refer the origin design doc on erigon, seems nothing clear about add_branch logic mentioned.

So, maybe we don't understand the logic currently (like some branch that did not generate updates were intentionally skipped), so can u plz offer some metarial to explain? Or hope this fix helps, we are willing to contribute to this repo constantly.

thanks,