Fix re-parameterize of MergeAppendPath

Instead of MergeAppendPath, MergeAppend nodes were considered.  This
code is not covered by any tests now, which should be addressed at some
point.

This is an oversight from f49842d, which introduced partition-wise joins
in v11, so back-patch down to that.

Author: Michael Paquier
Reviewed-by: Ashutosh Bapat
Discussion: https://postgr.es/m/20180718062202.GC8565@paquier.xyz

src/backend/optimizer/util/pathnode.c

@@ -3817,7 +3817,7 @@ do { \
 			}
 			break;
 
-		case T_MergeAppend:
+		case T_MergeAppendPath:
 			{
 				MergeAppendPath *mapath;