[MERGE] ParentProvider.get_parent_map()

[Aaron Bentley]

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John Arbash Meinel wrote:
> This updates all current parent providers to supply a 'get_parent_map()' function.
> 
> It also adds CachingParentProvider and several "XFAIL" tests.
> 
> It doesn't do any of the actual heads() or find_difference changes. I'm trying
> to break down my overall changes into simpler updates.

bb:approve

Looks great.  It might make sense to implement a ParentsProvider
superclass so that many of the implementations can share the same
get_parents implementation.

> I was thinking to deprecate "get_parents" on all of them, but that has to wait
> until the "one_one" deprecation strings go in.

Feel free.

Aaron
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