Using the `--state-branch` option to `git-filter-branch` to incrementally filter a branch

incremental-filter-branch.sh

#!/bin/bash
set -exuo pipefail

TRUNK=master
REWRITE_BRANCH=master-to-rewrite
NEW_BRANCH_BOOKMARK=master-filtered
LAST_IMPORT_BRANCH=master-lastimport
STATE=master-state
INDEX_FILTER_SCRIPT='git rm -r --cached --ignore-unmatch bad_directory'

# Update the trunk
git checkout -f $TRUNK
git clean -fd || true
git pull

# Prepare the branch to rewrite
git branch -D $REWRITE_BRANCH || true
git branch $REWRITE_BRANCH $TRUNK

# Go!
git filter-branch --index-filter "$INDEX_FILTER_SCRIPT" \
                  --state-branch $STATE \
                  --prune-empty \
                  -f \
                  $LAST_IMPORT_BRANCH..$REWRITE_BRANCH

git checkout $NEW_BRANCH_BOOKMARK
git merge --ff-only $REWRITE_BRANCH

# Cleanup
git checkout -f $LAST_IMPORT_BRANCH
git merge --ff-only $TRUNK
git checkout $TRUNK