gnu: magit-svn: Update to 2.1.0.

* gnu/packages/emacs.scm: Use (gnu packages base) module.  Remove
  duplicated (gnu packages guile) and (gnu packages linux) modules.
  (magit-svn): Update to 2.1.0.
  [source]: Use 'url-fetch' method.
  [inputs]: Remove.  Move "emacs" to 'native-inputs', move "magit" to
  'propagated-inputs'.
  [native-inputs]: Add "tar" and "gzip".
  [propagated-inputs]: Add "dash".
  [arguments]: Adjust builder for the new inputs and unpacking the source.