[emacs][clang-format] Add elisp API for clang-format on git diffs (#112792)
New proposed function `clang-format-vc-diff`.
It is the same as calling `clang-format-region` on all diffs between
the content of a buffer-file and the content of the file at git
revision HEAD. This is essentially the same thing as:
`git-clang-format -f {filename}`
If the current buffer is saved.
The motivation is many project (LLVM included) both have code that is
non-compliant with there clang-format style and disallow unrelated
format diffs in PRs. This means users can't just run
`clang-format-buffer` on the buffer they are working on, and need to
manually go through all the regions by hand to get them
formatted. This is both an error prone and annoying workflow.