This is the answer I received from the support today (2015-06-30):
Unfortunately we do not have a standalone installer at this time.
GitHub for Windows makes use of Microsoft’s ClickOnce technology for
installation and updates.We are currently working on an open source replacement for ClickOnce
here:https://github.com/squirrel/squirrel.windows
Once that technology is complete and ready for use, we hope to switch
GitHub for Windows to use that. It would allow for a standalone
installer.In the meantime, you can find a list of alternate GUIs available
here:http://git-scm.com/downloads/guis