[RFC] making the null revision become real

[Denys Duchier]

Currently bzr maintains a useful fiction that all projects are descended from
what is called the "null" revision.  However this is a virtual revision, and the
fact that it is not real, and therefore unlike other revisions, has undesirable
consequences:

    - it needs to be special-cased
    - asking for the "null" revision object raises an exception
    - None is also accepted to mean its revid and that clashes with the pythonic
      conventional value for omitted keyword arguments
    - revnos need to be offet by 1
    - it is not possible to do: "bzr init A; bzr branch A B"

I'd like to propose to turn the null revision into an actual revision.  It
should have revid "null:" i.e. bzrlib.revision.NULL_REVISION, and it should be
present as the first revision in every branch's history.  This means that revnos
no longer have to be offset; revno=0 is simply the null revision.  It also
becomes trivial to branch off a newly created empty branch.  And we should no
longer accept a revid=None to mean the null revision.  revid=None should only
mean that a revid has not been provided.

Opinions?

Cheers,

--Denys