Provide option to only give a warning instead of modifying the file

[andrewseidl]

This hook by default modifies files in place. Doing so will lead to commits with a mixture of formatting and code changes.

Instead, provide a warning saying that the files in the commit do not match the desired style. Optionally reject the commit.

[hugovdm]

Another option: git-clang-format is able to format only the changed lines, instead of the whole file.

(If I find the time and energy, I might eventually experiment with this a bit.)