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.
abort: repository a not found!
255
requesting all changes
abort: error: Connection refused
255
abort: repository a not found!
255
abort: destination '../a' already exists
1
abort: repository a not found!
255
abort: destination 'q' already exists