"The best way in my eyes is, to rebase because that fetches the latest changes of the upstream branch and replay your work on top of that." - Stefan Bauer
The ideal git history should have all Propeller modifications in a single most recent commit, followed by the full history from upstream CesiumGS.
e.g.
commit c6fc9cb205e387781c5269b95e350f61a8815d08
Author: Chris Cooper <chris@propelleraero.com>
Date: Mon Sep 21 15:19:02 2020 +1000
All the Propeller mods in a single commit
commit da4ddc58830606d89f762dc7d79b3dfe4103c6d1
Merge: 9c61508 6c0a0f8
Author: Kevin Ring <kevin@kotachrome.com>
Date: Tue Sep 1 15:47:38 2020 +1000
Updates for 1.73 release
...
To maintain this going forward, use a git rebase.
# Make sure your main branch is up to date
git checkout main
git pull
# Fetch the upstream tags
git fetch upstream --tags
# Interactive rebase to the CesiumGS release tag
# Squash all propeller commits into a single commit
git rebase -i 1.74
# Fix all the broken stuff :)
# You've just re-written the history so you'll have to force
git push origin main --force