Pairtree::Root.list doesn't work with 1-character directories

[cbeer]

This works:

it "should be the list of directories in the vault" do
subject.pairtree.mk('abc')
subject.pairtree.mk('def')
subject.pairtree.mk('ghi')
subject.pairtree.mk('jkl')
subject.pairtree.mk('mno')
subject.count.should == 5 # subject starts out empty
end

This doesn't:

it "should be the list of directories in the vault" do
subject.pairtree.mk('1')
subject.pairtree.mk('2')
subject.pairtree.mk('3')
subject.pairtree.mk('4')
subject.pairtree.mk('5')
subject.count.should == 5 # subject starts out empty
end