When git-clang-format is invoked with no explicit commit arguments and
both PRE_COMMIT_FROM_REF and PRE_COMMIT_TO_REF are set, the script
automatically uses those refs as the diff range and implies --diff. If
the variables are absent, existing behavior is fully preserved.
This allows projects to use `git-clang-format` directly inside CI
pipelines via the [pre-commit](https://pre-commit.com/) framework
without any wrapper scripts or extra configuration.
Closes: #188813
No existing lit test suite for this script. Verified manually that env
vars activate two-commit diff mode, existing behavior is preserved
without them, and explicit CLI args always override them.
ba44df4b88