git log and git diff can do everything you are describing here as far as comparisons.
git cherry-pick can apply any combination of changes we want to look at, only prompting to resolve any conflicts which should be minimal. This is how we’ve kept ClassicPress up to date with the WP 4.9 branch so far.
What’s still left to do is a way to track which changes we’ve accepted or rejected from other branches like 5.0, and link “merged” WP changesets together. This is what the web app described on the other thread will do. Until that’s ready we will mostly just keep up with the 4.9 branch which is easy.