match tipset getter code with Lotus

[LesnyRumcajs]

Summary of changes

Changes introduced in this pull request:

• This should match the Go code https://github.com/filecoin-project/lotus/blob/30a9f631653fed86072322b4dfccb31219ecb1f5/chain/stmgr/utils.go#L154-L163
• we eventually push this method to the FVM externs which caused state mismatch at epoch 2997616

Reference issue to close (if applicable)

Closes

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.