optional githook behaving as non-optional

I am attempting to make use of this gist in my workflow as post-merge and post-checkout git hooks.

#!/usr/bin/env bash
# MIT © Sindre Sorhus - sindresorhus.com

# git hook to run a command after `git pull` if a specified file was changed
# Run `chmod +x post-merge` to make it executable then put it into `.git/hooks/`.

changed_files="$(git diff-tree -r --name-only --no-commit-id ORIG_HEAD HEAD)"

check_run() {
    echo "$changed_files" | grep --quiet "$1" && eval "$2"
}

# Example usage
# In this example it's used to run `npm install` if package.json changed
check_run package.json "npm install"

This claims to only run npm install if the package.json file is changed.

However on all the machines I have tried this on. The npm install command runs regardless of whether package.json has been changed or not.

To test this I have been creating a new branch at my current commit and then checking it out, thus triggering the post-checkout git hook. I would not expect npm install to run because the package.json is unchanged.

via jth41