Patchwork [05,of,10,V2] branchmap: create a mercurial.branchmap module

# HG changeset patch
# User Pierre-Yves David <pierre-yves.david at ens-lyon.org>
# Date 1355924613 -3600
# Node ID a43f6900d52ce07f908fc49dcf96549a0db8cf07
# Parent  49e81a2c56e10861cc907520e734e63f02cbf18a
branchmap: create a mercurial.branchmap module

This is the foundation stone for an extraction of branches map logic from local
repository class.  Most of the branches map logic have very few caller and
therefor does not fit in the current criteria for code held by the localrepo
class. Important change will be made to this code in relation with revision
filtering. So we extract it in a dedicated module before adding additional
complexity.


Follow up commit do the actual code movement.