Extracting a custom module with a Git subtree

To extract a directory from a repository and keep the history, you can use the git subtree split command to create a new branch:

git subtree split --prefix=web/modules/custom/my_module --branch=split
Created branch 'split'
17835f24069061326e9e065f076afd67434e1b2f

This will create a new branch with just the contents of the given directory.

This can be pushed to a different repository:

git push git@github.com:opdavies/new-repo.git split:main