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.
#!/bin/sh
hg init
touch unknown
touch a
hg add a
hg ci -m "1" -d "0 0"
touch b
hg add b
hg ci -m "2" -d "0 0"
echo %% Should show unknown
hg status
hg revert -r 0
echo %% Should show unknown and b removed
hg status
echo %% Should show a and unknown
ls