lock: include Linux pid namespace identifier in prefix Previously, the lock only contains a hostname as an attempt to detect pid namespace difference. However, that's not enough on modern Linux - a single hostname could have different pid namespaces. That means if people run hg inside different PID namespaces with a same UTS namespae, the lock would be broken - an hg proccess in pid namespace A will think the lock having a "random" pid in pid namespace B is "dead" and remove it. This patch solves the above issue by appending an PID namespace identifier of the current process to the lock prefix ("hostname"). It depends on /proc being mounted properly. But I don't think there is a better way to get pid namespace identifier reliably.

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