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
#
mkdir t
cd t
hg init
echo 0 > a
echo 0 > b
hg ci -A -m m -d "0 0"
touch nottracked
hg locate a
hg locate NONEXISTENT
hg locate
hg rm a
hg ci -m m -d "0 0"
hg locate a
hg locate NONEXISTENT
hg locate
hg locate -r 0 a
hg locate -r 0 NONEXISTENT
hg locate -r 0