[clang-format] Fix the return code of git-clang-format

In diff and diffstat modes, the return code is != 0 even when there are no
changes between commits. This issue can be fixed by passing --exit-code to
git-diff command that returns 0 when there are no changes and using that as
the return code for git-clang-format.

Fixes #56736.

Differential Revision: https://reviews.llvm.org/D129311