Document git fork as "Pull-Request Only"

pr-only-fork.sh

#!/bin/bash
# Replace the repo's default branch with a README explaining this fork is only for its submitted pull requests.

set -e -x -o pipefail

REMOTE=origin
GH_USERNAME="${GH_USERNAME:-$USER}"

TRUNK="$(git symbolic-ref --short refs/remotes/$REMOTE/HEAD | sed "s:${REMOTE}/::")"
REMOTE_URL="$(git remote get-url $REMOTE)"
UPSTREAM_REPO="${REMOTE_URL%%.git}"

GIT_ROOT=$( git rev-parse --show-toplevel )
PROJECT_NAME="${GIT_ROOT##*/}"

# Move old trunk out of the way.
git branch -m "$TRUNK" upstream-"$TRUNK"

# Make a new orphan branch with that name.
git checkout --orphan "$TRUNK"

# checkout --orphan left everything staged in the index, remove it
git reset

# Remove most files, except for those that might cause a lot of editor thrashing
# or things the host needs for repo metadata (e.g. LICENSE)
git clean --force -d --exclude=.gitignore --exclude=.editorconfig --exclude=LICENSE

cat > README.rst << EOF
Pull-Request Only Fork
======================

This fork exists only for making pull requests against upstream.

See

* \`branches here <https://github.com/${GH_USERNAME}/${PROJECT_NAME}/branches>\`__
* \`upstream repository <${UPSTREAM_REPO}>\`__
* \`pull requests to upstream <${UPSTREAM_REPO}/pulls/${GH_USERNAME}>\`__
EOF