[MERGE] KnitGraphIndex and supporting work

[Aaron Bentley]

Aaron Bentley has voted +1.
Status is now: Approved
Comment:
I agree that a native implementation handling get_ancestry would be 
nice, but more, I think it would make sense to implement a breadth-first 
searcher on top of iter_all_entries.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C1184831530.11842.189.camel%40localhost.localdomain%3E