[MERGE] Replace VersionedFile.get_parents with get_parent_map

[Robert Collins]

This may seem large for a trivial change, but its not really that
trivial.

get_parents was a very old API from back with weaves and did work to
hide ghosts. So, removing it meant finding and fixing many places ghosts
were not handled correctly.

This patch then:
 - deprecates VersionedFile.get_parents
 - prohibits pulling a ghost containing repo into a non-ghost-capable
repo
 - adds VersionedFile.get_parent_map
 - tunes the internals of KnitVersionedFile to reduce friction and use
this api where possible

This is the first of many patches that will give us a much leaner
VersionedFile api, which I will then port across via a thunk layer to
VersionedFiles.

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