Ensure consistent sort order of large objects in pg_dump.

The primary purpose of this commit is to ensure pg_upgrade tests yield
comparable dumps pre/post upgrade, which got broken by 12a53c73 /
578b2297, as the order in pg_largeobject_metadata is likely to
differ pre/post upgrade.

It also seems like a generally good idea to make sure such dumps are
comparable, outside of pg_upgrade tests.

LO metadata already was already dumped in an ordered manner as the
metadata is dumped in a well defined order via
sortDumpableObjectsByTypeName() and sortDumpableObjects(). But large
object data is currently not tracked via that mechanism.

As Tom points out it seems possible that at some point dumpBlobs() was
assumed to dump out objects in a well defined order, due to the use of
DISTINCT, which at that time only was done using sorting.

Per complaint from Andrew Dunstan and discussion with him and Tom
Lane.

Author: Andres Freund
Discussion: https://postgr.es/m/2735.1543333649@sss.pgh.pa.us

src/bin/pg_dump/pg_dump.c

@@ -3328,9 +3328,13 @@ dumpBlobs(Archive *fout, void *arg)
 	 * the already-in-memory dumpable objects instead...
 	 */
 	if (fout->remoteVersion >= 90000)
-		blobQry = "DECLARE bloboid CURSOR FOR SELECT oid FROM pg_largeobject_metadata";
+		blobQry =
+			"DECLARE bloboid CURSOR FOR "
+			"SELECT oid FROM pg_largeobject_metadata ORDER BY 1";
 	else
-		blobQry = "DECLARE bloboid CURSOR FOR SELECT DISTINCT loid FROM pg_largeobject";
+		blobQry =
+			"DECLARE bloboid CURSOR FOR "
+			"SELECT DISTINCT loid FROM pg_largeobject ORDER BY 1";
 
 	ExecuteSqlStatement(fout, blobQry);