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 --debug init
echo this is a1 > a
hg add a
hg commit -m0 -d "0 0"
echo this is b1 > b
hg add b
hg commit -m1 -d "0 0"
hg manifest 1
echo this is c1 > c
hg rawcommit -p 1 -d "0 0" -m2 c
hg manifest 2
hg parents
rm b
hg rawcommit -p 2 -d "0 0" -m3 b
hg manifest 3
hg parents
echo this is a22 > a
hg rawcommit -p 3 -d "0 0" -m4 a
hg manifest 4
hg parents
echo this is c22 > c
hg rawcommit -p 1 -d "0 0" -m5 c
hg manifest 5
hg parents
# merge, but no files changed
hg rawcommit -p 4 -p 5 -d "0 0" -m6
hg manifest 6
hg parents
# no changes what-so-ever
hg rawcommit -p 6 -d "0 0" -m7
hg manifest 7
hg parents