revset: cache smartset's min/max

revset: cache smartset's min/max

As the content of a smartset never changes, min and max will never change
either.  This will save us time when this function is called multiple times.
This is relevant for issue4782 but does not fix it.

--- a/mercurial/revset.py	Fri Aug 28 10:53:55 2015 -0400
+++ b/mercurial/revset.py	Thu Aug 27 17:57:33 2015 -0700
@@ -2885,6 +2885,7 @@
         """True if the set will iterate in descending order"""
         raise NotImplementedError()
 
+    @util.cachefunc
     def min(self):
         """return the minimum element in the set"""
         if self.fastasc is not None:
@@ -2893,6 +2894,7 @@
             raise ValueError('arg is an empty sequence')
         return min(self)
 
+    @util.cachefunc
     def max(self):
         """return the maximum element in the set"""
         if self.fastdesc is not None: