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.

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#!/bin/sh

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hg init

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echo This is file a1 > a

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hg add a

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hg commit -m "commit #0" -d "0 0"

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ls

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echo This is file b1 > b

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hg add b

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hg commit -m "commit #1" -d "0 0"

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hg co 0

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# B should disappear

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ls