Further minor improvement in generic_xlog.c: always say REGBUF_STANDARD.
Further minor improvement in generic_xlog.c: always say REGBUF_STANDARD.

Since we're requiring pages handled by generic_xlog.c to be standard
format, specify REGBUF_STANDARD when doing a full-page image, so that
xloginsert.c can compress out the "hole" between pd_lower and pd_upper.
Given the current API in which this path will be taken only for a newly
initialized page, the hole is likely to be particularly large in such
cases, so that this oversight could easily be performance-significant.
I don't notice any particular change in the runtime of contrib/bloom's
regression test, though.

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