[MERGE][bug #235715] bzr merge --lca should handle NULL as a base

[John Arbash Meinel]

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The current --lca algorithm assumes that all common ancestors will be available
from the underlying versioned file. However, it is possible to have ancestries
that don't connect, causing NULL_REVISION to be an lca. And asking a VF for
NULL_REVISION raises a 'illegal' error.

This just updates the merge code so that if it gets NULL_REVISION it recognizes
that it should be treated as just an empty text.

I did this against 1.3, since that is what the bug was filed against, and what
is available in hardy. So it could easily go into a 1.3.2, or just into 1.6.
(The only issue is where to put the NEWS entry, whatever we decide to do.)

John
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