remove .git/index.lock': Permission denied

I had the exact same problem wanting to commit my changes to git, and solved it this way:

• I needed to change the group of my .git folder and its contents:

  sudo chown -R <owner>:<group> .git

• needed to change the permission of this folder:

  sudo chmod -R 775 .git

Check which user owns the git lock:

ls -la /Applications/MAMP/htdocs/my_site/.git/index.lock

Then you can use sudo to remove it.