[MERGE] use a serial number and one random value for fileids

[Robert Collins]

This patch uses a serial number and a single random value for 'bzr add'
rather than brand new randomness each time.

Its clearly still unique within a single process.

Globally it still needs a collision on the timestamp and 64 bits of
entropy to have a collision.

The only hole I can see is someone doing fork() after calling add, and
then calling add in the child. We probably should add an atfork hook to
reset the seed in this case.

Rob
-- 
GPG key available at: <http://www.robertcollins.net/keys.txt>.
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