I did see --more, but I don't think it suits this use case. I'm not looking for 10 or 30 or 100 additional commits; I'm looking for "however many are needed to cover all commits that differ between the branches."

ajv‭ What do you mean by that? Anything past the common ancestor, basically by definition, will not differ between the branches

After experimenting a bit, it looks like I only run into a problem if the tip of one branch is a merge of the other. e.g. consider a history like this:

master  *---*---A---B---*
             \           \
topic         a---b---c---*

If I then do git show-branch master topic, it doesn't include a, b, and c, even though those commits are not ancestors of master.

(updated post to include this)

ajv‭ Ah, merges do complicate things. I don't really think that there is a way around that though, at least with show-branch. The only workaround I can think of at the moment is using something like git show-branch master topic^ to essentially undo the merge in the eyes of git-show-branch, but that doesn't work for more than one merge and in any case probably has its own pitfalls.

If all you want is a pretty output, you could try using git log --graph?

git show-branch master topic^ is how I eventually dealt with the case I was facing, but as you say, that has its own issues. I do use git log --oneline --graph --boundary master...topic a lot, and that's often good enough, but for nontrivial histories it's hard to visually pick out which commits are on which branch.

I keep my own history rigidly clean, but my team are not git experts and sometimes break things when trying to clean their own. So I have to inspect messy histories more often than I'd like.