rfc: Change get_parent_map signature so that no parents is ()

[Aaron Bentley]

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John Arbash Meinel wrote:
> If we prefer, we could have it return 2 values
> ~  present_map, ghosts = get_XXXX()

I find multiple return values somewhat cumbersome, and it's clear that
the additional return value would not provide new information.  So I
would prefer to only have a map returned, unless checking for ghosts is
overwhelmingly common.

Aaron
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