Clarify why we can't do 'git rev-list' with a path.
lrwxrwxrwx 14 git-subtree -> git-subtree.sh
-rwxr-xr-x 8319 git-subtree.sh
-rw-r--r-- 23 shellopts.sh
-rwxr-xr-x 1989 test.sh
-rw-r--r-- 247 todo