Usage is intuitive; just click anywhere in a commit that isn't already a link, and the commit will load in place (unless already loaded) and unfold its whole changeset, or refold itself again, when clicked a second time. Similarly, individual files changed can be folded and unfolded much the same way, though only by clicking their headers (I often want to copy and paste stuff from diffs, so no click magic there). To unfold or refold every commit on the page, either hit "f", or click the "(un)fold all commits" link at the top or bottom right of the page.
This user script (install link, userscript.org page with docs and complete change log) is incidentally compatible with extensions like AutoPagerize too, in case you prefer scrolling for content to finding and clicking "Older" (h) and "Newer" (l). And if you just want to try it out without installing anything, try this bookmarklet on any github commits page.
And it's all done by a hundred lines of jQuery, about a dozen lines of css and another dozen lines of Greasemonkey / Chrome portability code that breaks out of their respective sandbox environments to run what the script needs to run in the context of the web page itself.
That latter piece is useful too, by the way, modeled on a trick Anthony Lieuallen came up with for the Greasemonkey wiki at some point. This is what it looks like, including full docs:
The above-referred jQuery is already loaded by github's page template, so I don't need to bother grabbing or shipping one of my own. If you are hacking a more hostile site, and run Greasemonkey you might want to add a
All above code is freely poachable (read: MIT license) at github, with as much or little attribution as you like. I would of course be especially happy to see the github people adopt this for their default page templates, as it's really useful stuff. Share and enjoy!