[MERGE] Reduce key lookups during commit

[Robert Collins]

We have a bit of a mispattern at the moment.

It goes like this:
 - generate unique id
 - insert a text with that id
   - check that the id is unique

This is double-handling, so this patch removes that, when we have
signalled that the id is unique, we trust it to be so.

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