[MERGE] Accept NULL_REVISION everywhere

[Aaron Bentley]

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Hi all,

This patch causes many, many functions to accept NULL_REVISION as their
input, in addition to None.  It is phase 1 of a project to remove all
use of None to represent the null revision.

Using None to indicate the null revision means that we can't use it to
mean "no value supplied".

Well, it means we shouldn't.  In fact, I have found several places where
None was being supplied to indicate the null revision, but was being
interpreted as "no value supplied".

In phase 2, I will change Branch.last_revision and other methods to
return NULL_REVISION where appropriate.  I plan to do phase 2 for Bazaar
0.20, since this does break the API.

Plugins that wish to anticipate this change can do so by wrapping
Branch.last_revision() with bzrlib.revision.ensure_null.

Aaron
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