Hmm... can't actually filter rev-list on the subdir name.
tree8592a69af509d2d22629be51f5fc6bb8239c4e33
-rw-r--r-- 3 .gitignore
lrwxrwxrwx 14 git-subtree -> git-subtree.sh
-rwxr-xr-x 4828 git-subtree.sh
-rw-r--r-- 23 shellopts.sh