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
echo "Is there an error message when trying to diff non-existing files?"
hg diff not found
echo "Is there an error message when trying to add non-existing files?"
hg add not found