Checkout PR, using GitHub API to determine what to track

checkout_pr.sh

#!/bin/sh
set -o errexit
[ $# != 1 ] && { >&2 echo 'Argument: <pull request id>'; exit 1; }

# Get origin remote
origin=$(git remote get-url origin)

# Find target ("owner/repo") of PR
temp=${origin#*github.com?}
[ $origin = $temp ] && { >&2 echo Unrecognized origin.; exit 1; }
target=${temp%.git}

# Find source ("owner/repo") of PR and remote name and branch name
temp=$(curl --silent https://api.github.com/repos/$target/pulls/$1 | perl -MJSON::PP -0777 -E '$head=decode_json(<>)->{head}; say $head->{repo}->{full_name} . ":" . $head->{ref}')
source=${temp%%:*}
remote=$(echo $origin | sed "s $target $source ")
branch=${temp#*:}

# Set up branch and checkout
git fetch --no-tags $remote $branch:pr/$1
git config branch.pr/$1.remote $remote
git config branch.pr/$1.merge refs/heads/$branch
git checkout pr/$1

Updated again to use JSON::PP to actually parse the JSON.