engine/src/build/git-hooks/pre-commit

#!/bin/sh

submodule_diff() {
  if test -n "$2"; then
    git diff-tree -r --ignore-submodules=dirty "$1" "$2" | grep -e '^:160000' -e '^:...... 160000' | xargs
  else
    git diff-index --cached --ignore-submodules=dirty "$1" | grep -e '^:160000' -e '^:...... 160000' | xargs
  fi
}

if git rev-parse --verify --quiet --no-revs MERGE_HEAD; then
  merge_base=$(git merge-base HEAD MERGE_HEAD)
  if test -z "$(submodule_diff $merge_base HEAD)"; then
    # Most up-to-date submodules are in MERGE_HEAD.
    head_ref=MERGE_HEAD
  else
    # Most up-to-date submodules are in HEAD.
    head_ref=HEAD
  fi
else
  # No merge in progress. Submodules must match HEAD.
  head_ref=HEAD
fi

submods=$(submodule_diff $head_ref)
if test "$submods"; then
  echo "You are trying to commit changes to the following submodules:" 1>&2
  echo 1>&2
  echo $submods | cut -d ' ' -f 6 | sed 's/^/  /g' 1>&2
  cat <<EOF 1>&2

Submodule commits are not allowed.  Please run:

  git status --ignore-submodules=dirty

and/or:

  git diff-index --cached --ignore-submodules=dirty HEAD

... to see what's in your index.

If you're really and truly trying to roll the version of a submodule, you should
commit the new version to DEPS, instead.
EOF
  exit 1
fi

gitmodules_diff() {
  git diff-index --cached "$1" .gitmodules
}

if [ "$(git ls-files .gitmodules)" ] && [ "$(gitmodules_diff $head_ref)" ]; then
  cat <<EOF 1>&2
You are trying to commit a change to .gitmodules.  That is not allowed.
To make changes to submodule names/paths, edit DEPS.
EOF
  exit 1
fi

exit 0