Not surprisingly, since it'd never ever been tested, ExecReScanGatherMerge
didn't work.  Fix it, and add a regression test case to exercise it.

Amit Kapila

Discussion: https://postgr.es/m/CAA4eK1JkByysFJNh9M349u_nNjqETuEnY_y1VUc_kJiU0bxtaQ@mail.gmail.com