Handle it successfully if a given parent commit has no parents.
treea93c5217f09e3bebe2cbc9180d73fe2c3c0e39f6
-rw-r--r-- 3 .gitignore
lrwxrwxrwx 14 git-subtree -> git-subtree.sh
-rwxr-xr-x 8171 git-subtree.sh
-rw-r--r-- 23 shellopts.sh
-rwxr-xr-x 1926 test.sh