[MERGE/RFC] Shortcut 'iter_references' when we can't have references

[Aaron Bentley]

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Ian Clatworthy wrote:
> John Arbash Meinel wrote:
>> In playing around with merging, I found that the "WT.iter_references()"
>> call was showing up somewhat heavily in the time for doing a merge.
> 
> I found the same thing when profiling branch on large projects.
> In the case of branch though, IIRC the slow iter_references call was
> being done in bzrdir.sprout() and the method was on DirStateRevisionTree,
> not workingtree_4.

Strangely, I didn't have to include that change to get equivalent
performance to your patch.

>> Thoughts? It seems a shame to penalize (heavily) our normal code because
>> we might be supporting references in some formats (that aren't even
>> considered production worthy yet.)
> 
> That was my thinking too. I'm not close enough to the subtree-related
> code to know how safe making this change is

I think it's a good idea.

Aaron
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