smartset: convert set to list lazily

If the caller only wants to construct a baseset via a set, and then do
"__contains__" tests. It's unnecessary to initialize the list.

Testing on my unfiltered hg-committed repo where len(draft()) is 2600, this
patch shows about 6% improvement on set intensive queries:

Before:

  $ for i in `seq 5`; hg perfrevset 'draft() & draft() & draft() & draft()'
  ! wall 0.001196 comb 0.000000 user 0.000000 sys 0.000000 (best of 2011)
  ! wall 0.001191 comb 0.000000 user 0.000000 sys 0.000000 (best of 2099)
  ! wall 0.001186 comb 0.010000 user 0.010000 sys 0.000000 (best of 1953)
  ! wall 0.001182 comb 0.000000 user 0.000000 sys 0.000000 (best of 2135)
  ! wall 0.001193 comb 0.000000 user 0.000000 sys 0.000000 (best of 2177)

After:

  $ for i in `seq 5`; hg perfrevset 'draft() & draft() & draft() & draft()'
  ! wall 0.001128 comb 0.000000 user 0.000000 sys 0.000000 (best of 2247)
  ! wall 0.001119 comb 0.000000 user 0.000000 sys 0.000000 (best of 2317)
  ! wall 0.001115 comb 0.000000 user 0.000000 sys 0.000000 (best of 2244)
  ! wall 0.001131 comb 0.000000 user 0.000000 sys 0.000000 (best of 2093)
  ! wall 0.001124 comb 0.000000 user 0.000000 sys 0.000000 (best of 2134)

It could have bigger impact on larger sets in theory.

mercurial/smartset.py

@@ -171,7 +171,11 b' class baseset(abstractsmartset):'
                 self._set = data
                 # set has no order we pick one for stability purpose
                 self._ascending = True
+                # converting set to list has a cost, do it lazily
+                data = None
+            else:
             data = list(data)
+        if data is not None:
         self._list = data
         self._datarepr = datarepr
 
@@ -185,6 +189,12 b' class baseset(abstractsmartset):'
         asclist.sort()
         return asclist
 
+    @util.propertycache
+    def _list(self):
+        # _list is only lazily constructed if we have _set
+        assert '_set' in self.__dict__
+        return list(self._set)
+
     def __iter__(self):
         if self._ascending is None:
             return iter(self._list)
@@ -204,7 +214,7 b' class baseset(abstractsmartset):'
         return self._set.__contains__
 
     def __nonzero__(self):
-        return bool(self._list)
+        return bool(len(self))
 
     def sort(self, reverse=False):
         self._ascending = not bool(reverse)
@@ -218,7 +228,10 b' class baseset(abstractsmartset):'
         self._istopo = False
 
     def __len__(self):
+        if '_list' in self.__dict__:
         return len(self._list)
+        else:
+            return len(self._set)
 
     def isascending(self):
         """Returns True if the collection is ascending order, False if not.