[MERGE/RFC] MemoryTree and a TreeBuilder helper

[Robert Collins]

This is the start of another pass at test-suite performance, its been a
year since my last such pass.

This time round my goal is to get closer to, or even reach, having all
tests that are not testing workingtree specifically stop hitting local
disk.

I've introduced MemoryTree which is a MutableTree that caches no data
and is backed by memory. Common code with WorkingTree has been pulled up
into the new MutableTree common parent.

TreeBuilder is where I plan to move a number of test tweaking facilities
- different styles of building trees etc. I considered doing it as a
decorator, but decided that for the moment having it just a separate
object was cleaner. Its probable that benchmarks.tree_create.TreeCreator
should be a subclass of TreeBuilder, or something like that.

As to what performance changes I expect - I've only converted one test
so far, as proof of concept - and that improved by 25%.


I'm interested in working on this incrementally, I'm going to convert a
couple of tests a day, extending the support api as needed.

For now - I'd love to get a +1 on the current code, or feedback
sufficient to get that.

Cheers,
Rob



-- 
GPG key available at: <http://www.robertcollins.net/keys.txt>.
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