Add this to .git/hooks/pre-commit and make sure it's executable.

Objective-c auto formatting git hook

#!/bin/bash

# brew install clang-format

STYLE=$(git config --get hooks.clangformat.style)
if [ -n "${STYLE}" ] ; then
  STYLEARG="-style=${STYLE}"
else
  STYLEARG="-style=file"
fi

format_file() {
  file="${1}"
  if [[ $file =~ .*\.[mh]$ ]];then
    clang-format -i ${STYLEARG} $file
    git add ${1}
  fi
}

case "${1}" in
  --about )
    echo "Runs clang-format on source files"
    ;;
  * )
    for file in `git diff-index --cached --name-only HEAD` ; do
      format_file "${file}"
    done
    ;;
esac