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 123 > a
echo 123 > c
hg add a c
hg commit -m "first" -d "0 0" a c
echo 123 > b
echo %% should show b unknown
hg status
echo 12 > c
echo %% should show b unknown and c modified
hg status
hg add b
echo %% should show b added and c modified
hg status
hg rm a
echo %% should show a removed, b added and c modified
hg status
hg revert a
echo %% should show b added and c modified
hg status
hg revert b
echo %% should show b unknown and c modified
hg status
hg revert c
echo %% should show b unknown
hg status
echo %% should show a b and c
ls
true