> Instead of "priming the reader" it's infinitely more helpful to tell the reader why you did something, because you can't extract that from a diff.
Again, that can go in the PR body or in subsequent lines. You have ~50 characters in that first line, which is never going to be enough to fully explain anything.
I'm also not suggesting that you eliminate the PR body: that should also include more context. All I'm suggesting is that taking the trouble to organize your commits into discrete units helps reviewers to understand how you perceive the various changes in a single PR as being related to one another, and no amount of text in the PR body will provide the same benefit as being able to look at several distinct diffs containing related changes.
I'm trying to imagine the near infinite terms I would have to search for to find the commit where I "changed from a hash to a set".
Regardless, every other thing you said could also just be done in the central PR body (and thus the merge commit) and be much easier to access.
Instead of "priming the reader" it's infinitely more helpful to tell the reader why you did something, because you can't extract that from a diff.