Source: Checking out pull requests locally – GitHub Help
Working for Overviewer:
git fetch origin pull/1640/head:scaffolding
In my dual remote working copy:
[remote “origin”]
url = git@github.com:IncredibleHolg/Minecraft-Overviewer.git
fetch = +refs/heads/*:refs/remotes/origin/*
[remote “main”]
url = https://github.com/overviewer/Minecraft-Overviewer.git
fetch = +refs/heads/*:refs/remotes/main/*
git fetch main pull/1640/head:scaffolding