/u/samatman is definitely mistaken, and I think you are correct (although I'm not sure about "the set of changes since that commit"). As far as I can tell, `git diff <commit>` and `git diff <branch>` both just diff your current workspace against the commit/branch respectively.

In the case of the branch, the correct result is something like "what has changed since my branch diverged from its parent"--basically what you see in a PR. I think this is unnecessarily obscure in Git because a "branch" isn't really a branch in the git data model; rather it's something like "the tip of the branch".

I don't think I've ever wanted to compare my workspace against a branch, but clearly diffing the branch is useful (as evidenced by PRs). Similarly, I'm much less inclined to diff my workspace against a particular commit, but I often want to see the contents of an individual commit (another common operation in the Github UI).

In essence, if Github is any indicator, Git's data model is a subpar fit for the VCS use case.