Yeah, this is easier ... and broken down ... Work on a temporary branch, get stuff all working there, keep merging changes elsewhere into the temporary branch

Yeah, this is easier ... and broken down ... Work on a temporary branch, get stuff all working there, keep merging changes elsewhere into the temporary branch to stay up to date with your PR target branch. Push the temporary branch early and often and don't worry too much about its commit messages (will never be part of official history) except as they pertain to your understanding of the evolution. Then create a new branch (for PR) off the one you'll eventually PR to, and `merge --squash` your temporary branch into the branch for PR with an awesome commit message. Submit the PR. Kill your temporary branch when no longer useful. Modify the strategy a bit if you want to break up the PR into multiple coarse commits ... 100 temporary-branch commits, 3 new-material commits in for-PR branch, and 4 back-merges of PR target branch to for-PR branch into temporary branch. This keeps a full context of your changes vs upstream changes.