[PATCH 4 of 8 clfilter-part1] clfilter: handle non contiguous iteration in `revlov.headrevs`

[pierre-yves.david at logilab.fr]

# HG changeset patch
# User Pierre-Yves David <pierre-yves.david at logilab.fr>
# Date 1346674365 -7200
# Node ID 90376bfc2f426a89571a1d6cb05ad068a4f512c1
# Parent  2127a50786c9a765776e77044d3e9fa3790ec6ff
clfilter: handle non contiguous iteration in `revlov.headrevs`

This prepares changelog level filtering.  We can't assume that any revision can
be heads because filtered revisions need to be excluded.

New algorithm:
- All revisions now start as "non heads",
- every revision we iterate over is made candidate head,
- parents of iterated revisions are definitely not head.

Filtered revisions are never iterated over and never considered as candidate
head.

diff --git a/mercurial/revlog.py b/mercurial/revlog.py
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -609,16 +609,18 @@ class revlog(object):
         except AttributeError:
             pass
         count = len(self)
         if not count:
             return [nullrev]
-        ishead = [1] * (count + 1)
+        # we won't iter over filtered rev so nobody is a head at start
+        ishead = [0] * (count + 1)
         index = self.index
         for r in self:
+            ishead[r] = 1  # I may be an head
             e = index[r]
-            ishead[e[5]] = ishead[e[6]] = 0
-        return [r for r in xrange(count) if ishead[r]]
+            ishead[e[5]] = ishead[e[6]] = 0  # my parent are not
+        return [r for r, val in enumerate(ishead) if val]
 
     def heads(self, start=None, stop=None):
         """return the list of all nodes that have no children
 
         if start is specified, only heads that are descendants of