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.

%% no changes
%% no changes
%% uncompressed contents in repo
this is a test
%% uncompress our working dir copy
this is a test
%% uncompress our new working dir copy
this is a test