[PATCH 1 of 5] ancestor: use absolute_import

[Gregory Szorc]

# HG changeset patch
# User Gregory Szorc <gregory.szorc at gmail.com>
# Date 1439001948 25200
#      Fri Aug 07 19:45:48 2015 -0700
# Branch stable
# Node ID b743eb4d51b0e9ec0b0ffbc269a8d1b2163727a5
# Parent  79f0cb97d7537a7c2948f8f9b0a89148825a3a1d
ancestor: use absolute_import

A few months ago, import-checker.py was taught to enforce a more
well-defined import style for files with absolute_import. However,
we stopped short of actually converting source files to use
absolute_import because of problems with certain files.

Investigation revealed the following problems with switching to
absolute_import universally:

1) import cycles result in import failure on Python 2.6
2) undetermined way to import C/pure modules

While these problems need to be solved, they can be put off.
This patch starts a series of converting files to absolute_import
that won't exhibit any of the aforementioned problems.

diff --git a/mercurial/ancestor.py b/mercurial/ancestor.py
--- a/mercurial/ancestor.py
+++ b/mercurial/ancestor.py
@@ -4,11 +4,14 @@
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
+from __future__ import absolute_import
+
 import collections
 import heapq
-from node import nullrev
+
+from .node import nullrev
 
 def commonancestorsheads(pfunc, *nodes):
     """Returns a set with the heads of all common ancestors of all nodes,
     heads(::nodes[0] and ::nodes[1] and ...) .