[MERGE REQUEST] updated merge base picker to handle multiple roots

[Aaron Bentley]

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Hi Martin,

Please merge http://panoramicfeedback.com/opensource/bzr.24

It fixes the base ancestor picker by using the null revision as the
graph root.

It also introduces a revision id for the null revision, 'null:'.  I
believe that having a real rev id for the null revision is better,
because it clarifies that the null revision is being intentionally
specified, and it frees up None to mean 'no revision matched the
requirements' in return values and 'not specified' in arguments.

The value is specified in bzrlib.revision.NULL_REVISION, so you can
change this to another string if you prefer, or None if you don't want
to allow an id for the null revision.

Aaron
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