Fixed regression with splitting out new subtree

A folder in a repository that wasn't initially imported as a  subtree could no longer be splitted into an entirely new subtree with no parent.

A fix and a new test to fix that regression is added here.
diff --git a/git-subtree.sh b/git-subtree.sh
index b7c3501..a86cfd8 100755
--- a/git-subtree.sh
+++ b/git-subtree.sh
Binary files differ