git-breakout

#!/bin/bash

set -Eeuo pipefail

if [ $# -lt 2 ]; then
  cat <<-EOT
	Usage: git breakout <branch_name> <commit>...

	    This command creates a PR that contains some commits taken from the
	    current branch (or anywhere else, really) and applied on top of the current
	    branch's upstream branch.

	    The broken out commits will form a new branch which will only get a branch
	    head in the »origin« repository, but not in the local repository.

	    This command operates directly on git objects, isolated from the current branch,
	    working tree and index, so you can use it a any point,
	    and it's faster than rebasing.

	    The drawback is that if any commit's patch doesn't apply cleanly, it will
	    make the whole operation fail, as there's no mechanism to resolve conflicts.

	    <commit> may be given as either individual commits or commit ranges.

	    There are almost no sanity checks, use at your own risk!

	    Good luck!
	EOT
  exit 1
fi

BRANCH=$1
shift

SIGN_FLAG=
if [[ $(git config --get --type bool commit.gpgSign) == true ]]; then
  SIGN_FLAG=-S
fi

GIT_INDEX_FILE=$(git rev-parse --git-dir)/breakout-index
export GIT_INDEX_FILE
trap "rm $GIT_INDEX_FILE" EXIT

PREV=$(git rev-parse origin/HEAD)
git read-tree $PREV

for COMMIT in $(git rev-list --reverse --no-walk "$@"); do
  git show -s --format='Applying %t %s' $COMMIT
  if ! git diff-tree -p $COMMIT | git apply --cached; then
    echo Could not apply changes....
    echo Aborting...
    exit 1
  fi

  TREE=$(git write-tree)
  PREV=$(git show -s --format='%B' $COMMIT | env \
    GIT_AUTHOR_NAME="$(git show -s --format='%an' $COMMIT)" \
    GIT_AUTHOR_EMAIL="$(git show -s --format='%ae' $COMMIT)" \
    GIT_AUTHOR_DATE="$(git show -s --format='%ad' $COMMIT)" \
    git commit-tree $SIGN_FLAG -p $PREV $TREE)
done

git push origin $PREV:refs/heads/${BRANCH};

if command -v gh &>/dev/null; then
  gh pr create -w --head ${BRANCH}
else
  echo GitHub CLI not installed, please manually open the web interface to create the PR
fi

exit 0