tuple header.  The fix is based on the thought that HEAP_MOVED_IN is
not needed any more as soon as HEAP_XMIN_COMMITTED has been set.  So
in tqual.c and vacuum.c the HEAP_MOVED bits are cleared when
HEAP_XMIN_COMMITTED is set.

Vacuum robustness is enhanced by rearranging ifs, so that we have a
chance to elog(ERROR, ...) before an assertion fails.

A new regression test is included.

Manfred Koizar