[MERGE] trees - add get_parent_ids

[Aaron Bentley]

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Robert Collins wrote:
> This patch adds a get_parent_ids() call to all our tree objects (at
> least, I think to all of them ;)).
> 
> This call returns:
> [] for a tree with unknown parents

I think this should be something other than the empty list.  Possibly an
exception.

> ['null:'] for a tree descending from null:

e.g. revno 1, right?

> [revid, ...] in general.
> 
> EmptyTree descends from ['null:'].

EINFINITELOOP.  EmptyTree doesn't descend from 'null:', it *is* 'null:'.
 EmptyTree should return [], because 'null:' does not descend from 'null:'.

> RevisionTree mirrors the revision graph.
> WorkingTree provides the union of last_revision and pending merges.

These are fine, though.

Aaron
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