[PATCH 04 of 10 V2] discovery: move common heads computation inside partialdiscovery object

[Boris Feld]

# HG changeset patch
# User Boris Feld <boris.feld at octobus.net>
# Date 1545964082 -3600
#      Fri Dec 28 03:28:02 2018 +0100
# Node ID b70041eb7b51f23c10732138c7254468f8232f55
# Parent  36fd99749d7bb9acd2122dc909d27dc7e54dde8d
# EXP-Topic discovery-refactor
# Available At https://bitbucket.org/octobus/mercurial-devel/
#              hg pull https://bitbucket.org/octobus/mercurial-devel/ -r b70041eb7b51
discovery: move common heads computation inside partialdiscovery object

This remove one of the private attribute access. In additions, head tracking
and computation is a typical operation we can speed up using Rust.

diff --git a/mercurial/setdiscovery.py b/mercurial/setdiscovery.py
--- a/mercurial/setdiscovery.py
+++ b/mercurial/setdiscovery.py
@@ -182,6 +182,10 @@ class partialdiscovery(object):
         """return True is we have any clue about the remote state"""
         return self._common.hasbases()
 
+    def commonheads(self):
+        """the heads of the known common set"""
+        return set(self._repo.revs('heads(%ld)',
+                   self._common.bases - {nullrev}))
 
 def findcommonheads(ui, local, remote,
                     initialsamplesize=100,
@@ -314,7 +318,7 @@ def findcommonheads(ui, local, remote,
     # heads(common) == heads(common.bases) since common represents common.bases
     # and all its ancestors
     # The presence of nullrev will confuse heads(). So filter it out.
-    result = set(local.revs('heads(%ld)', disco._common.bases - {nullrev}))
+    result = disco.commonheads()
     elapsed = util.timer() - start
     progress.complete()
     ui.debug("%d total queries in %.4fs\n" % (roundtrips, elapsed))