[MERGE] Fix RemoteRepository.get_parent_map stacking

[Aaron Bentley]

Hi all,

This patch fixes a significant bug in RemoteRepository stacking:
get_parent_map did not make use of fallback repositories.

This meant that a push could easily wind up pushing almost the entire
history into a stacked branch, even though the stacked_on branch already
had that history.  This was probably the root cause of bug #293679.

Aaron
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