`git diff <commit>' is the set of changes since that commit, not the changes of that commit (the distinction between hg diff -r and hg diff -c). Similarly, `git diff <branch>' is the diff between that branch and the current HEAD, not the diff of the branch itself.
So perhaps it's a great example if you've gotten it wrong?
/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.
That's fair, insofar as I'm unfamiliar with the mercurial commands.
It's unfair insofar as it does what I expect it to, which is to diff between what I'm curious about, and where I am.
In other words, if you elide the second argument, it defaults to wherever HEAD is.
The point being, this is not something I personally need to look up. I'd venture a guess that your familiarity with hg is interfering because the conventions are different.
That brings up a deeper issue with git's philosophy. Git's UI is largely geared to introspecting the repository history only insofar as it exists to the currently existing checkout--commands that interact with history without concerning themselves with the current checkout are far more inscrutable, confusing, and difficult to find.
By contrast, Mercurial's UI makes the repository history a more first-class citizen, and it is very easy to answer basic questions about the history of the repository itself. If you're doing any sort of source code archaeology, that functionality is far more valuable than comparing it to the current state: I don't want to know what changed since this 5-year-old patch, I want to know what this 5-year-old patch itself changed to fix an issue.
> I'd venture a guess that your familiarity with hg is interfering because the conventions are different.
Git users also need to answer questions like "What changes are in my feature branch?" (e.g., a PR) and "What changed in this commit?" (e.g., GitHub's single-commit-diff view). These aren't Mercurial-specific questions, they're applicable to all VCSes including Git, as evidenced by the (widely-used) features in GitHub.
Even with Git, I've never wanted to know how my workspace compares to another branch, nor how a given commit compares to my workspace (except when that commit is a small offset off my workspace).
> In other words, if you elide the second argument, it defaults to wherever HEAD is.
Yeah, I get that, but that's not helpful because I still need to calculate the second argument. For example, `git diff master..feature-branch` is incorrect, I want something like `git diff $(git merge-base master feature-branch)..feature-branch` (because the diff is between feature-branch and feature-branch's common ancestor with master, not with HEAD of master).
One of the cool things about Mercurial is it has standard selectors for things. `hg log -b feature-branch` will return just the log entries of the range of commits in the feature-branch (not their ancestors in master, unlike `git log feature-branch`). Similarly, `-c <commit>` always returns a single-commit range (something like <commit>^1..<commit> in git). It's this consistency and sanity in the UI that makes Mercurial so nice to work with, and which allows me to recall with better accuracy the hg commands that I used >5 years ago than the git commands that I've used in the last month.
So perhaps it's a great example if you've gotten it wrong?