add prechangegroup and pretxnchangegroup hooks. prechangegroup lets you stop push, pull or unbundle before it begins. pretxnchangegroup lets you inspect changegroup before transaction is committed, and roll back if you not like it.

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# HG changeset patch
# User #author#
# Date #date|date#
# Node ID #node#
#parent%changesetparent#
#desc#

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