In this case, the specific error was:
$ git pull
error: unable to unlink old 'sites/default/default.settings.php': Permission denied
I opened the sites directory and saw the following:
$ lll
rwxr-xr-x noxlady staff 160 B Tue Sep 4 13:34:39 2018 M? all/
r-xr-xr-x noxlady staff 192 B Tue Sep 4 13:40:49 2018 ✓ default/