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

[via cfe-commits]

goldsteinn wrote:

Current return from `package-lint`:

```
3 issues found:

1:0: error: Package should have a Homepage or URL header.
1:60: warning: You should depend on (emacs "24.1") if you need lexical-binding.
223:19: error: You should depend on (emacs "25.1") if you need `vc-root-dir'.
```

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