Added a --onto option, but it's so complicated I can't tell if it works.
tree7694cdb39fa3afffa9187ee96c910cbebfb40ac9
-rw-r--r-- 3 .gitignore
lrwxrwxrwx 14 git-subtree -> git-subtree.sh
-rwxr-xr-x 4710 git-subtree.sh
-rw-r--r-- 23 shellopts.sh