Back-patch fix for timestamp(datetime) into 6.5.*.

Back-patch fix for timestamp(datetime) into 6.5.*.
I believe this is not a violation of our policy against requiring initdb
for minor-version bugfixes, since users don't *have* to initdb; they
just won't see any effect from the fix if they don't.  But a user who
does do a clean install or initdb upgrade to 6.5.2 will get the fix,
and that seems worthwhile.