fetchfossil: Require rev.

Otherwise no bumps are done when upstream changes. Not sure how to ping the
repository and check without just doing the clone.

pkgs/build-support/fetchfossil/default.nix

@@ -1,6 +1,6 @@
 {stdenv, fossil}:
 
-{name ? null, url, rev ? null, md5 ? null, sha256 ? null}:
+{name ? null, url, rev, md5 ? null, sha256 ? null}:
 
 stdenv.mkDerivation {
   name = "fossil-archive" + (if name != null then "-${name}" else "");