lib/attrset: optimize element access in recursiveUpdateUntil

- Eta reduce formal arguments of `recursiveUpdate'.
- Access elements in `recursiveUpdateUntil` using `elemAt` and `head`
  directly instead of `head (tail xs)` which copies a singleton unnecessarily.
  (`elemAt` is used instead of `last` to save a primitive call to `length`,
  this is possible because the 2-tuple structure is guranteed)
- Use `length` instead of comparison to empty list to save a copy.

lib/attrsets.nix

@@ -419,8 +419,8 @@ rec {
     let f = attrPath:
       zipAttrsWith (n: values:
         let here = attrPath ++ [n]; in
-        if tail values == []
-        || pred here (head (tail values)) (head values) then
+        if length values == 1
+        || pred here (elemAt values 1) (head values) then
           head values
         else
           f here values
@@ -446,10 +446,7 @@ rec {
        }
 
      */
-  recursiveUpdate = lhs: rhs:
-    recursiveUpdateUntil (path: lhs: rhs:
-      !(isAttrs lhs && isAttrs rhs)
-    ) lhs rhs;
+  recursiveUpdate = recursiveUpdateUntil (path: lhs: rhs: !(isAttrs lhs && isAttrs rhs));
 
   /* Returns true if the pattern is contained in the set. False otherwise.