[MERGE] Implement Tree.plan_merge, deprecate RevisionTree.get_weave

[John Arbash Meinel]

John Arbash Meinel has voted +1 (conditional).
Status is now: Conditionally approved
Comment:
+1 modulo the discussion between you and Robert about how to get the 
parents correct.

I think you actually want to report the parent ids as all ids except for 
the one you are merging. I believe I understand your point that if you 
have the parent_id included, then merge has nothing to do, because OTHER 
is now part of THIS.

However, it would seem that annotation-based merge when you have 
multiple merges already, should include all parents (but the current one 
you are re-evaluating).

Then again, I thought 'bzr remerge' was disabled in the case of having 
 >1 merge parents.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C469F8927.1080705%40utoronto.ca%3E