[clang] [emacs][clang-format] Add elisp API for clang-format on git diffs (PR #112792)

[via cfe-commits]

goldsteinn wrote:

> > I don't disagree these are all potential pitfalls (and there are certainly more), I just don't see how having the diff code in a separate project ameliorates any of them. And as stated earlier, I think it in fact complicates them.
> 
> The issue of "complexity" is quite subjective, a customizable function that returns line ranges doesn't strike me as a complex API that's likely to break/change with the benefit of easily integrating other diffing methods.
> 
> From a user perspective it likely just means one extra package, possibly setting a configuration value.
> 
> Or, to avoid this PR having to handle system & version-control spesific details - we could consider calling `vc-diff` directly to generate the diff - this abstracts away all the details of calling git & diff directly, personally I would still rather keep the functionality separate - then this can be easily integrated with `hl-diff` or other packages that already track changed lines, removing the overhead of the current method used.

`vc-diff` AFAICT won't work because it operates on files not buffer contents. It would be a nice simplifcation, although doesn't really change the fact we depend on `vc`.

All things considered, I think requiring `vc` is a lot less intrusive than essentially our own `vc` wrapper....

https://github.com/llvm/llvm-project/pull/112792