Introduce an interface for DirectoryFiller to allow testing

In JGitFS I tried to unit test some of my code, but failed to create a unit test for the readdir() implementation because DirectoryFiller is an implementation class which cannot be instantiated at all from outside of fuse-jna. 

This patch introduces an interface DirectoryFiller and moves the implementation into an implementation class. This way I can create a dummy implementation for my unit tests. No actual code-change takes place, only moving of actual code into new class.