reproduce (this is a W=1 build):
        # https://github.com/koverstreet/bcachefs/commit/3f4c3f5210af2d5f2e472c4e1eb7dda3e1518d4d
        git remote add koverstreet-bcachefs https://github.com/koverstreet/bcachefs
        git fetch --no-tags koverstreet-bcachefs bcachefs-testing
        git checkout 3f4c3f5210af2d5f2e472c4e1eb7dda3e1518d4d
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=x86_64 olddefconfig
        make W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash fs/bcachefs/