Correctly assign a_value for return of Chef::ChefFS::FileSystem#self.compare

Previously this was breaking the output of higher abstractions like Chef::ChefFS::CommandLine#diff_entries and friends.
Both the `a.compare_to(b)` and `b.compare_to(a)` statements were overwriting one of the return values (to `nil`) on each call.
That was compounded by the second `if are_same.nil?` statement, resulting in the `*_value` statement never getting returned when evaluating 2 Chef::ChefFS entries.