[MERGE] Only loop graph.iter_ancestry once when running "bzr log FILE"

[Aaron Bentley]

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Gary van der Merwe wrote:
> I thought this was going to increase the speed of "bzr log FILE" alot,
> but it only made it a bit faster. I did not realize that the loading
> of the graph is cached.
> 
> lifeless said I should still submit.
> 
> Quote from Irc:
> <lifeless>	 the index layer does have a cache of its own, and the
> majority of the cost of traversing is disk io and parsing, yes.
> <lifeless>	 but with a big enough data set, the cache can be
> exhausted, which would lead to duplicate IO

Also, Graph instances will have their own cache via
CachingParentsProvider, and that will not expire for the lifetime of the
Graph.

Aaron
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