TL;DR For a single repo, add a special “refspec” for a remote (e.g. upstream):

git config --add remote.upstream.fetch "+refs/pull/*/head:refs/pullreqs/*"

which will allow you to do things like this:

leo@conservation:spectre> git fetch upstream
remote: Enumerating objects...
...
 * [new ref]               refs/pull/6218/head -> refs/pullreqs/6218
leo@conservation:spectre> git log refs/pullreqs/6218 # Or git diff, any git command
commit 5de7c8b1c54df99587f3a2e923e99b8db7a16f9f (HEAD)
Author: Carter Anderson <cra3@williams.edu>
Date:   Wed Aug 14 13:35:23 2024 -0400

    Add option for higher dimension EOSes to Spec initial data

...
leo@conservation:spectre> git checkout refs/pullreqs/6218
leo@conservation:spectre> git switch -c workOnHelpingThisPR

and then you’ll be at work on top of this PR. This means you don’t have to go find and add the remote for this PR.

I learned this magic refspec from the forge package for magit (which is itself an emacs package). If you want to become one with git, I highly recommend magit.

Save for future convenience

You can make yourself a global git alias to automate this, so you don’t have to remember the magic incantation for the above refspec. Run the following one-liner (though it is a very long line):

git config --global alias.addFetchGithubPRs '!MYREMOTE=${1:-origin}; if git remote get-url "$MYREMOTE" >/dev/null 2>&1; then if git config --get-all remote."$MYREMOTE".fetch | grep -q "+refs/pull/\*/head:refs/pullreqs/\*"; then echo Found correct refspec; else git config --add remote."$MYREMOTE".fetch "+refs/pull/*/head:refs/pullreqs/*"; echo Added refspec; fi ; else echo Remote not found: "$MYREMOTE"; fi #'

Now from inside your favorite repo, you can do:

leo@conservation:myrepo> git addFetchGithubPRs [remoteName]
Added refspec

The default remoteName is origin, but by changing the alias above, you can make it be something else (like upstream). If the remote doesn’t exist, you get an error. If the refspec already existed, it will tell you that.