Use unsigned version format. This way can use one additional bit, and when encountering invalid revlogs with the first bit set don't produce python warnings or strange error messages.

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#!/bin/sh

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# http://www.selenic.com/mercurial/bts/issue433

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hg init a

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cd a       

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echo a > a 

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hg commit -Ama 

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hg parents -r 0 doesnotexist

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true