Is descent git branch

is_descent_branch.sh

pushedrev=$1
basename=${2:-develop}

if ! baserev="$(git rev-parse --verify refs/heads/"$basename" 2>/dev/null)"; then
    echo "'$basename' is missing, call for help!"
    exit 1
fi

parents_of_commits_beyond_base="$(
  git rev-list --pretty=tformat:%P "$pushedrev" --not "$baserev" |
  grep -v '^commit '
)"

case "$parents_of_commits_beyond_base" in
    *\ *)          echo "$pushedrev must not push merge commits (rebase instead)"
                   exit 1 ;;
    *"$baserev"*)  exit 0 ;;
    *)             echo "$pushedrev must descend from tip of '$basename'"
                   exit 1 ;;
esac

# usage example: ~/scripts/is_descent_branch.sh child-name parent-branch
# Read more about it in https://stackoverflow.com/a/3162929/3454593