Fix #1023: mailmerge of person queries grouped by some attribute

[tbar0970]

As you note your your code, the "re-flattening" is a bit yucky. I've solved it differently in https://github.com/tbar0970/jethro-pmm/commit/feb6642c1d33e2e0bf80b1b9701e1f4146da4bbb by making sure it's a flat result in the first place.

[jefft]

Clever! I've verified that fixes the bug.