diffstat: clean up expression

Most importantly, add meta.description and meta.platforms.

pkgs/tools/text/diffstat/default.nix

···
-{fetchurl, stdenv}:
+{ fetchurl, stdenv }:
 
 stdenv.mkDerivation rec {
   name = "diffstat-1.58";
 
   src = fetchurl {
-    url = "ftp://invisible-island.net/diffstat/"+ name +".tgz";
+    url = "ftp://invisible-island.net/diffstat/${name}.tgz";
     sha256 = "14rpf5c05ff30f6vn6pn6pzy0k4g4is5im656ahsxff3k58i7mgs";
   };
 
-  meta = {
+  meta = with stdenv.lib; {
+    description = "Read output of diff and display a histogram of the changes";
+    longDescription = ''
+      diffstat reads the output of diff and displays a histogram of the
+      insertions, deletions, and modifications per-file. It is useful for
+      reviewing large, complex patch files.
+    '';
     homepage = http://invisible-island.net/diffstat/;
-    longDescription = "diffstat reads the output of diff and displays a
-istogram of the insertions, deletions, and modifications per-file. It
-s useful for reviewing large, complex patch files.";
+    license = licenses.mit;
+    platforms = platforms.linux;
+    maintainers = [ maintainers.bjornfor ];
   };
 }