Make limit/2 more efficient

Contributed by @pkoppstein.

Author: nicowilliams

src/builtin.jq

@@ -168,13 +168,9 @@ def until(cond; next):
          if cond then . else (next|_until) end;
      _until;
 def limit($n; exp):
-  if $n < 0 then exp else
-    label $out |
-    foreach exp as $item (
-      [$n, null];
-      if .[0] < 1 then break $out
-        else [.[0] -1, $item] end;
-      if .[0] == 0 then .[1], break $out else .[1] end) end;
+  if $n < 0 then exp
+  else label $out | foreach exp as $item ($n; .-1; $item, if . <= 0 then break $out else empty end)
+  end;
 def isempty(g): 0 == ((label $go | g | (1, break $go)) // 0);
 def first(g): label $out | g | ., break $out;
 def last(g): reduce g as $item (null; $item);