AreOnTheSameFork: don't actually need to find common ancestor

Summary:
We don't actually need to know what is the common ancestor (which may
require iterating over many blocks below pa and pb). It's enough to
just perform the initial phase from LastCommonAncestor, which is
what this new code does.

Test Plan: ninja check-all

Reviewers: jasonbcox, #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5068