[MERGE] Fix bad ghost handling in _KnitParentsProvider

[Andrew Bennetts]

bb:approve

Aaron Bentley wrote:
> Hi all,
> 
> _KnitParentsProvider.get_parent_map had a small flaw with ghost
> handling-- it would try to assign parents to ghosts.  If the ghost was
> the first in the list, it would fail with a traceback, because the name
> 'parents' wasn't defined.  If the ghost came later, I believe it would
> reuse the parents from the previous request.
> 
> This patch fixes it.

Good catch.  Your fix is obviously correct, please merge it.

> In the process, I noticed that some implementations of get_parents_map
> return mappings where the values are lists, and for other
> implementations, the values are tuples.  Can we standardize please?  I
> suggest tuples.

FWIW, I'd be happy with tuples.

-Andrew.