[MERGE] Multiple element keys.

[Robert Collins]

This adds the ability to do a GraphIndex with keys like (FILEID,
REVISION), which is a key element that I need to complete the
elimination of .kndx files in the experimental repository format, and
will make the first round of repository refactoring feature complete.

Its essentially two changes:
 - index keys go from bytestrings to tuples of bytestrings
 - add a iter_entries_prefix method to GraphIndex indices allowing for
the equivalent of 'iter_all_entries' for one FILEID using the above
mapping.

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