Consolidate tx rehashing in helper function as expected

Summary: Rehashing the tx in the helper method makes it's usage less error-prone

Test Plan: test_runner.py bip65-cltv-p2p

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2071