Git hooks to setup upstream automatically for a PR. Doc included within the script.

post-checkout

#!/bin/sh
# Automatically wire up push config for branches named `pull/<N>` so that a
# plain `git push` on a PR branch pushes back to the contributor's fork.
#
# Requires: gh (GitHub CLI, authenticated) and jq.
# The contributor must have "Allow edits by maintainers" enabled on the PR.
#
# Args passed by git: $1=prev_head $2=new_head $3=branch_checkout_flag (1 = branch)
# Can also be invoked manually (no args) from inside a repo while on a
# `pull/<N>` branch — useful to (re)run the setup without re-checkout.
#
# ---------------------------------------------------------------------------
# Setup on a new machine
# ---------------------------------------------------------------------------
# 1. Install prerequisites:
#      macOS:   brew install gh jq
#      Linux:   apt install gh jq   (or your distro equivalent)
# 2. Authenticate gh:
#      gh auth login
# 3. Drop this file at ~/.githooks/post-checkout and make it executable:
#      mkdir -p ~/.githooks
#      cp post-checkout ~/.githooks/post-checkout
#      chmod +x ~/.githooks/post-checkout
# 4. Point every repo at this shared hooks directory:
#      git config --global core.hooksPath ~/.githooks
# 5. Allow `git push` to use the upstream branch name even when it differs
#    from the local branch name (required because `pull/6701` tracks e.g.
#    `contributor/fix/theme-toggle-reload`):
#      git config --global push.default upstream
#
# ---------------------------------------------------------------------------
# Per-repo configuration so PRs appear as `pull/<N>`
# ---------------------------------------------------------------------------
# By default `git fetch` does not retrieve GitHub's pull-request refs. Add
# this refspec to the origin remote so every PR is fetched as
# `refs/remotes/origin/pull/<N>`:
#
#     git config --add remote.origin.fetch '+refs/pull/*/head:refs/remotes/origin/pull/*'
#     git fetch origin
#
# After that you can `git checkout pull/<N>` to land on any PR branch, and
# this hook will prompt to wire up push config for it.
#
# Resulting `.git/config` fragment:
#     [remote "origin"]
#         url = git@github.com:<org>/<repo>.git
#         fetch = +refs/heads/*:refs/remotes/origin/*
#         fetch = +refs/pull/*/head:refs/remotes/origin/pull/*

set -eu

log() { printf 'post-checkout: %s\n' "$*" >&2; }

# When invoked as a git hook, git passes 3 args and $3=1 for branch checkouts.
# When invoked manually (no args) from inside a repo, skip that check.
if [ $# -ge 3 ]; then
    [ "$3" = "1" ] || exit 0
fi

if ! branch=$(git symbolic-ref --short -q HEAD); then
    log "HEAD is not on a branch (detached?); nothing to configure"
    exit 0
fi

case "$branch" in
    pull/[0-9]*) ;;
    *)
        log "branch '$branch' does not match 'pull/<number>'; nothing to configure"
        exit 0
        ;;
esac

num=${branch#pull/}

# Skip if we've already configured this branch.
if git config --get "branch.$branch.pushRemote" >/dev/null; then
    exit 0
fi

if ! command -v gh >/dev/null 2>&1; then
    log "'gh' not found in PATH; skipping PR push config for '$branch'"
    exit 0
fi
if ! command -v jq >/dev/null 2>&1; then
    log "'jq' not found in PATH; skipping PR push config for '$branch'"
    exit 0
fi

# Ask the user before touching remotes / config. Read from the controlling tty
# so this works even when the hook's stdin isn't interactive (e.g. under gh/git
# wrappers). If there is no tty, skip silently.
if [ ! -r /dev/tty ] || [ ! -w /dev/tty ]; then
    log "no controlling tty; skipping interactive setup for '$branch'"
    exit 0
fi

printf '\npost-checkout: set up push to the contributor fork for PR #%s? [y/N] ' "$num" >/dev/tty
read -r answer </dev/tty || exit 0
case "$answer" in
    y|Y|yes|YES) ;;
    *) log "skipped setup for '$branch'"; exit 0 ;;
esac

data=$(gh pr view "$num" --json headRefName,headRepository,headRepositoryOwner 2>/dev/null) || {
    log "failed to fetch PR #$num metadata via gh; is this a GitHub repo and are you authenticated?"
    exit 0
}

owner=$(printf '%s' "$data" | jq -r '.headRepositoryOwner.login // empty')
repo=$(printf  '%s' "$data" | jq -r '.headRepository.name // empty')
head=$(printf  '%s' "$data" | jq -r '.headRefName // empty')

if [ -z "$owner" ] || [ -z "$repo" ] || [ -z "$head" ]; then
    log "incomplete PR metadata for #$num (owner='$owner' repo='$repo' head='$head'); aborting"
    exit 0
fi

if ! git remote get-url "$owner" >/dev/null 2>&1; then
    git remote add "$owner" "git@github.com:$owner/$repo.git"
    log "added remote '$owner' -> git@github.com:$owner/$repo.git"
fi

if ! git fetch --quiet "$owner" "$head"; then
    log "failed to fetch $owner/$head; leaving branch config untouched"
    exit 0
fi

git config "branch.$branch.remote"     "$owner"
git config "branch.$branch.pushRemote" "$owner"
git config "branch.$branch.merge"      "refs/heads/$head"

log "configured '$branch' to push to $owner/$repo:$head"