merger: don't create key_def/merger for one source #226
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A merge source has the same API as merger itself, so there is no
difference in behaviour between a merger created from one source and
this source itself. However there is no overhead for creating key_def,
merger and passing tuples over the merger.
This optimization gives me 13% boost on the case from #220.
Follows up #220
There is no simple way to check it with a test, so I skipped it. Hopefully we'll see a difference after #225. I tested the patch manually for now.