[MERGE] Enable merging PreviewTrees

[Aaron Bentley]

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Robert Collins wrote:
> On Wed, 2008-10-08 at 23:11 -0400, Aaron Bentley wrote:
>>
>> This patch adds support for merging PreviewTrees into other trees.
>> This
>> means changing some methods to avoid using Tree.inventory, which
>> PreviewTree does not supply.
>>
>> In the process, it introduces Tree.get_inventory_entry, which provides
>> a
>> convenient way of getting single inventory entries, without actually
>> using Tree.inventory or Tree.iter_entries_by_dir.  This is a new
>> requirement for Tree implementors, but Tree provides a default for
>> subclasses.
> 
> I don't understand why this is preferred to accessing the inventory :-
> this new method is asking for and returning an inventory entry object;
> the natural home for that appears to be on the inventory object itself.

Inventory objects are meant to be an implementation detail of certain
Tree types.  Hitting the inventory is expensive on DirState-based trees,
and we've worked hard to eliminate .inventory access from our key
codepaths.  PreviewTree is the next step, because it eliminates the
.inventory attribute entirely.

Aaron
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