Disable abbreviated keys on Windows.

Most of the Windows buildfarm members (bowerbird, hamerkop, currawong,
jacana, brolga) are unhappy with yesterday's abbreviated keys patch,
although there are some (narwhal, frogmouth) that seem OK with it.
Since there's no obvious pattern to explain why some are working and
others are failing, just disable this across-the-board on Windows for
now.  This is a bit unfortunate since the optimization will be a big
win in some cases, but we can't leave the buildfarm broken.

src/backend/utils/adt/varlena.c

@@ -1744,14 +1744,22 @@ static void
 btsortsupport_worker(SortSupport ssup, Oid collid)
 {
 	TextSortSupport	   *tss;
+	bool				abbreviate = ssup->abbreviate;
 
 	/*
 	 * WIN32 requires complex hacks when the database encoding is UTF-8 (except
 	 * when using the "C" collation).  For now, we don't optimize that case.
+	 * The use of abbreviated keys is also disabled on Windows, because
+	 * strxfrm() doesn't appear to work properly on some Windows systems.
+	 * Ideally, we would use it on those systems where it's reliable and
+	 * skip it only for the rest, but at the moment we don't know how to
+	 * distinguish between the ones where it works and the ones where it
+	 * doesn't.
 	 */
 #ifdef WIN32
 	if (GetDatabaseEncoding() == PG_UTF8 && !lc_collate_is_c(collid))
 		return;
+	abbreviate = false;
 #endif
 
 	/*
@@ -1838,7 +1846,7 @@ btsortsupport_worker(SortSupport ssup, Oid collid)
 	else
 		ssup->abbrev_full_comparator = ssup->comparator = bttextfastcmp_locale;
 
-	if (!lc_collate_is_c(collid) || ssup->abbreviate)
+	if (!lc_collate_is_c(collid) || abbreviate)
 	{
 		/*
 		 * Abbreviated case requires temp buffers for strxfrm() copying.
@@ -1852,7 +1860,7 @@ btsortsupport_worker(SortSupport ssup, Oid collid)
 		ssup->ssup_extra = tss;
 	}
 
-	if (!ssup->abbreviate)
+	if (!abbreviate)
 		return;
 
 	initHyperLogLog(&tss->abbr_card, 10);