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

[Aaron Bentley]

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

> 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.

Did you consider just doing it in Tree.add?  That seems even safer and 
more transparent to me.

Aaron