git-spinoff

#! /bin/sh

# git-spinoff: spin current work off into a new branch
#
# usage is:
#
#    git spinoff <branchname>
#
# which creates a new branch named <branchname> pointing to the
# current commit, ready for new commits if the index and/or work
# tree are dirty, and restoring the current branch to its upstream,
# provided that we are in fact on a branch now that has an upstream.

die() {
    echo "fatal: $@" 1>&2
    exit 1
}

# check arguments
case $# in
1) newbranch="$1";;
*) echo "usage: git spinoff <newbranch>" 1>&2; exit 1;;
esac

# make sure we are on a branch that has an upstream
fullbranch=$(git symbolic-ref -q HEAD) ||
    die "existing branch is detached; there's nothing to restore"
branch=${fullbranch#refs/heads/}
upstream=$(git rev-parse -q --verify @{u}) ||
    die "existing branch $branch has no upstream: there's nowhere to restore-to"

# now create and check out out the new branch, or quit if we can't
git checkout -b "$newbranch" || exit $?

# last, re-adjust the previous branch (which is no longer the current
# branch since we are on the new one we created) to point to its own
# upstream (if this fails, ignore the failure!)
git update-ref -m \
    "git spinoff: re-point $branch to ${branch}@{upstream}" \
    $fullbranch $upstream