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/
  rw-r--r--  noxlady  staff  904 B    Tue Sep  4 13:34:38 2018   ✓    README.txt
  rw-r--r--  noxlady  staff    2 KiB  Tue Sep  4 13:34:39 2018   ✓    example.sites.php
So I don't have write permissions for the "default" directory...
$ chmod 755 default 
And now I see:
$ lll
  rwxr-xr-x  noxlady  staff  160 B    Tue Sep  4 13:34:39 2018  M?    all/
  rwxr-xr-x  noxlady  staff  192 B    Tue Sep  4 13:40:49 2018   ✓    default/
  rw-r--r--  noxlady  staff  904 B    Tue Sep  4 13:34:38 2018   ✓    README.txt
  rw-r--r--  noxlady  staff    2 KiB  Tue Sep  4 13:34:39 2018   ✓    example.sites.php
Alas, the failed git pull left a whole bunch of clutter in my folder, so I need to git reset --hard HEAD and  git clean -f -d to clear out all the changes...
Now I can git pull successfully!
THX!