In fact, a very easy way to get the dev branch from within StaSh is to run the following one-line command (without editing any dot files):

SELFUPDATE_BRANCH=dev selfupdate

A bit more explanations on it how works:

SELFUPDATE_BRANCH is an environment variable to the shell and has default value of master, i.e. selfupdate by default grab the master branch. Like any other shells, environment variable can be set as VARNAME=value. The above command temporarily override the default value and make selfupdate grab the dev branch.