Split a Git directory into a different repo with history

subtree_split.bash

# there are probably more efficient ways to do this, but this seems clear and light on low-level commands
# it also does all operations locally until ready to push

cd /path/to/orig_repo

# split off directory into a branch
git subtree split --prefix path/to/subdirectory --branch subdirectory-split

cd /path/to/new_repo

# merge the branch into the new repo
git remote add orig_repo-local /path/to/orig_repo
git fetch orig_repo-local subdirectory-split
# git merge --allow-unrelated-histories orig_repo-local/subdirectory-split

# however, subtree split & merge doesn't seem to allow merging to the, or any, prefix directory
# so this low-level nonsense is required to put the subtree back in the right place
git merge --allow-unrelated-histories -s ours --no-commit orig_repo-local/subdirectory-split
git read-tree --prefix path/to/subdirectory -u orig_repo-local/subdirectory-split
git commit -m 'Merge subdirectory from orig_repo'