[MERGE] handle null revision properly for LCA

[Aaron Bentley]

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Hi all,

The new Graph methods require that the null revision be expressed as
'null:' not None.  Using None has turned out to be a bad idea in our
APIs, so I propose that all new APIs accept only 'null:'.

This patch fixes input to find_unique_lca, so that it doesn't get None.
 Previously, it would go into an infinite loop if None was provided.
This patch prevents infinite looping-- it raises an exception instead.

Aaron
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