Optimized the incremental update case.

This brings the incremental closer to a regular query (about 1.5 times
as bad instead of 3.5 times).
For a comparison look at MailQueryBenchmark::testIncremental()

The optimization is built on the assumption that we i.e. get an update
with 100 revisions, and thus the optimization applies to the case where
we have multiple revisions within that batch that are part of the same
reduction. In such a case we can avoid redoing the reduction lookup over
and over.

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