fetchgitiles: add tag argument

pkgs/build-support/fetchgitiles/default.nix

@@ -3,24 +3,36 @@
 lib.makeOverridable (
   {
     url,
-    rev,
+    rev ? null,
+    tag ? null,
     name ? "source",
     ...
   }@args:
 
+  assert (
+    lib.assertMsg (lib.xor (tag == null) (
+      rev == null
+    )) "fetchFromGitiles requires one of either `rev` or `tag` to be provided (not both)."
+  );
+
+  let
+    realrev = (if tag != null then "refs/tags/" + tag else rev);
+  in
+
   fetchzip (
     {
       inherit name;
-      url = "${url}/+archive/${rev}.tar.gz";
+      url = "${url}/+archive/${realrev}.tar.gz";
       stripRoot = false;
       meta.homepage = url;
     }
     // removeAttrs args [
       "url"
+      "tag"
       "rev"
     ]
   )
   // {
-    inherit rev;
+    inherit rev tag;
   }
 )