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 a
hg add a
hg commit -m "Added a" -d "0 0"
touch main
hg add main
hg commit -m "Added main" -d "0 0"
hg checkout 0
echo Main should be gone
ls
touch side1
hg add side1
hg commit -m "Added side1" -d "0 0"
touch side2
hg add side2
hg commit -m "Added side2" -d "0 0"
hg log
echo Should have two heads, side2 and main
hg heads
echo Should show "a side1 side2"
ls
hg update --debug -C 1
echo Should only show "a main"
ls