Since Git 1.7.10, separate configuration files can be included from ~/.gitconfig:
The configuration mechanism learned an "include" facility; an
assignment to the include.path pseudo-variable causes the named
file to be included in-place when Git looks up configuration
It may be interesting to write about it in README.markdown to allow people not on Mac OS X to safely put their main Git config in a dotfiles-like repos and their credentials in a private separate config file.
Related but not exactly the same is #92 by @MindTooth.
We're now exclusively using OAuth tokens for authentication. You can use gist --login to get a new token :).