Don't use abbreviated keys for the final merge pass.

Don't use abbreviated keys for the final merge pass.

When we write tuples out to disk and read them back in, the abbreviated
keys become non-abbreviated, because the readtup routines don't know
anything about abbreviation.  But without this fix, the rest of the
code still thinks the abbreviation-aware compartor should be used,
so chaos ensues.

Report by Andrew Gierth; patch by Peter Geoghegan.