revset: added lazyset class with basic operations

This class allows us to return values from large revsets as soon as they are
computed instead of having to wait for the entire revset to be calculated.

mercurial/revset.py

@@ -2072,5 +2072,23 b' class baseset(list):'
         l = [r for r in x if r not in s]
         return baseset(list(self) + l)
 
+class lazyset(object):
+    """Duck type for baseset class which iterates lazily over the revisions in
+    the subset and contains a function which tests for membership in the
+    revset
+    """
+    def __init__(self, subset, condition):
+        self._subset = subset
+        self._condition = condition
+
+    def __contains__(self, x):
+        return x in self._subset and self._condition(x)
+
+    def __iter__(self):
+        cond = self._condition
+        for x in self._subset:
+            if cond(x):
+                yield x
+
 # tell hggettext to extract docstrings from these functions:
 i18nfunctions = symbols.values()