revset: don't cache abstractsmartset min/max invocations infinitely stable

revset: don't cache abstractsmartset min/max invocations infinitely

There was a "leak", apparently introduced in ab66c1dee405. When running:

    hg = hglib.open('repo')
    while True:
        hg.log("max(branch('default'))")

all filteredset instances from branch() would be cached indefinitely by the
@util.cachefunc annotation on the max() implementation.

util.cachefunc seems dangerous as method decorator and is barely used elsewhere
in the code base. Instead, just open code caching by having the min/max
methods replace themselves with a plain lambda returning the result.

--- a/mercurial/revset.py	Mon Oct 24 09:14:34 2016 -0500
+++ b/mercurial/revset.py	Tue Oct 25 18:56:27 2016 +0200
@@ -2924,23 +2924,29 @@
         """True if the set will iterate in topographical order"""
         raise NotImplementedError()
 
-    @util.cachefunc
     def min(self):
         """return the minimum element in the set"""
-        if self.fastasc is not None:
-            for r in self.fastasc():
-                return r
-            raise ValueError('arg is an empty sequence')
-        return min(self)
-
-    @util.cachefunc
+        if self.fastasc is None:
+            v = min(self)
+        else:
+            for v in self.fastasc():
+                break
+            else:
+                raise ValueError('arg is an empty sequence')
+        self.min = lambda: v
+        return v
+
     def max(self):
         """return the maximum element in the set"""
-        if self.fastdesc is not None:
-            for r in self.fastdesc():
-                return r
-            raise ValueError('arg is an empty sequence')
-        return max(self)
+        if self.fastdesc is None:
+            return max(self)
+        else:
+            for v in self.fastdesc():
+                break
+            else:
+                raise ValueError('arg is an empty sequence')
+        self.max = lambda: v
+        return v
 
     def first(self):
         """return the first element in the set (user iteration perspective)