Pre-commit hook does not stash unstaged changes before running code style checkers

How to reproduce:

1. Make some changes to C files and violate whitespace rules.
2. git add affected files and try to git commit. Pre-commit hook will not allow it and will print the whitespace issues it found.
3. Fix whitespace problems, but forget to git add the files.
4. Running git commit again does not reject the changes, despite whitespace fixes not being staged. New commit now includes whitespace violations and none of the fixes that were done in step 3.

This is not limited to whitespace issues, but could affect other code style checks as well.