Remove leading '/' from path in FileSystemBackend

When doing `git clone git://localhost/foo.git` I get an `Invalid path /foo.git` exception even though `self.root = '/home/git/repositories'`. This is because the path given is `/foo.git` and that makes the call to `os.path.join` have no effect.

Don't know if I'm using git clone the wrong way, but this PR fixes this issue when using `dulwich daemon`.