Adjust DatumGetBool macro, this time for sure.
Adjust DatumGetBool macro, this time for sure.

Commit 23a41573c attempted to fix the DatumGetBool macro to ignore bits
in a Datum that are to the left of the actual bool value.  But it did that
by casting the Datum to bool; and on compilers that use C99 semantics for
bool, that ends up being a whole-word test, not a 1-byte test.  This seems
to be the true explanation for contrib/seg failing in VS2015.  To fix, use
GET_1_BYTE() explicitly.  I think in the previous patch, I'd had some idea
of not having to commit to bool being exactly 1 byte wide, but regardless
of what the compiler's bool is, boolean columns and Datums are certainly
1 byte wide.

The previous fix was (eventually) back-patched into all active versions,
so do likewise with this one.

jmcatamney/gpdb,jmcatamney/gpdb,xinzweb/gpdb,50wu/gpdb,xinzweb/gpdb,greenplum-db/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,50wu/gpdb,jmcatamney/gpdb,lisakowen/gpdb,xinzweb/gpdb,lisakowen/gpdb,adam8157/gpdb,50wu/gpdb,50wu/gpdb,adam8157/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,xinzweb/gpdb,50wu/gpdb,adam8157/gpdb,jmcatamney/gpdb,lisakowen/gpdb,ashwinstar/gpdb,50wu/gpdb,lisakowen/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,lisakowen/gpdb,greenplum-db/gpdb,lisakowen/gpdb,ashwinstar/gpdb,50wu/gpdb,adam8157/gpdb,adam8157/gpdb,adam8157/gpdb,adam8157/gpdb,ashwinstar/gpdb,adam8157/gpdb,jmcatamney/gpdb,xinzweb/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,jmcatamney/gpdb,50wu/gpdb,ashwinstar/gpdb,lisakowen/gpdb