I'm ashamed to admit that there's a fair few things on git that I've just simply never learned - I've taken the hackish route around a problem and just never stopped to consider that there might be a better way.
One of those is looking at pull requests for Mumble. I don't have any sort of privileges on the mainline Mumble source repos, and I don't know if Github makes it easier for folks who do. In the past when someone's made a pull request I wanted to look at...
... I would clone their entire repo and switch to the branch, then build it. :(
It turns out there's a far easier way:
git fetch upstream pull/1540/head:test_overlay_changes git checkout test_overlay_changes
... and now I can take advantage of having Mumble half-built already. I'm ashamed that I'd not figured this out earlier, and after doing a Google search for a decent source for this information it turns out Github even have a help page on the matter.