you might wonder why these are different from all the other variables in
the rc file... it's just that I never thought people would want to
change these!
- allow a mob username to be defined; all unauthenticated access will
look to gitolite like this user (if you setup apache also properly)
- update doc with more details (some repeat stuff from `man
git-http-backend` but it's probably worth having everything in one
place