[MERGE] Reduce index query pressure for text construction by batching the individual queries into single batch queries.

[Robert Collins]

This not-quite-trivial change pushes the index queries made to build
texts down into each knit index layer, reducing the number of queries by
2 (method + parents + location -> one query) and also batches all the
requested components into a single query rather than
one-loop-per-text-being-built, which will help things like 'extract two
inventories'.

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