Memoirs of a QWERTY Keyboard
Just like it did when I upgraded from OS X Mavericks to OS X Yosemite a year ago, attempting an upgrade from OSX Yosemite to OSX El Capitan hung again in the same nondescript manner - a blank/empty screen with the pointer, for seemingly infinite period.
When you are dealing with a local git repository which has multiple remotes defined (i.e. has multiple remote repositories possibly carrying the same branches, such as multiple forks), then the right way to checkout a working branch from one of them is to use the below syntax:
git checkout -b master remote-name/master