Problem with an automated review: clang-format requests whitespace changes even when files already conform to Google C++ Style Guide

[karlt]

Phabricator URL: https://phabricator.services.mozilla.com/D15802#402212

Problem:
Automated reviews that detect when code doesn't follow Google C++ Style Guide would be useful but the review bot is using clang-format to go beyond that and enforce unspecified requirements.

I assume that additional undocumented preferences to choose one single output are not a goal, and https://bugzilla.mozilla.org/show_bug.cgi?id=1188202#c7 indicates that clang-format is not idempotent, which means clang-format would not be appropriate for such a goal anyway.

Can clang-format be run in a way that only applies its own preferences when it encounters lines that do not conform to the Google C++ Style Guide?