git-overwrite-branch.sh

#!/bin/bash

die () {
    echo "$0: $@" > /dev/stderr
    exit 1
}

usage () {
    echo "Usage: $0 <branchname> <directory>" > /dev/stderr
    exit 1
}

[ -z "$(git status --porcelain)" ] || \
    die "must be run with clean working directory"

branch="$1"
git show-ref --verify --quiet "refs/heads/$branch" || usage

dir="$2"
[ -e "$dir" ] || usage

[ "$(git rev-parse --show-toplevel)" = "$(pwd)" ] || \
    die "must be run from top directory of project"

newbranch="$branch-new-contents"
current="$(git rev-parse --abbrev-ref HEAD)"

set -e

git checkout "$branch"
git merge -s ours -m merge "$current"

git checkout -b "$newbranch"
git merge -s ours -m merge "$branch"
git add -f "$dir"
git commit -m "update"
git filter-branch --subdirectory-filter "$dir" -- HEAD^!
git update-ref -d "refs/original/refs/heads/$newbranch"

git checkout "$branch"
git merge --squash "$newbranch"
git commit -m "update"
git branch -D "$newbranch"

git checkout "$current"