Fix #113: Fix whitespace in filenames in diff

The 'diff.py' module used to parse unified diff lines like

  +++ a/foo/bar /spam/eggs

as

  "/foo/bar"

i.e. stripping anything behind whitespace. This is because it tried to
recognize the part after whitespace as a commit revision id. As dulwich
never uses the latter, I simply removed that feature. Whitespace isn't
special-cased anymore now.