Report diff in block mismatch

Include the diff between the first mismatching byte in the log output
as we have it, and it can aid debugging (it just now helped me). Can't
hurt.

contrib/gp_replica_check/gp_replica_check.c

@@ -345,6 +345,7 @@ retry:
 		char		mirrorFileBuf[BLCKSZ];
 		int			primaryFileBytesRead;
 		int			mirrorFileBytesRead;
+		int			diff;
 
 		CHECK_FOR_INTERRUPTS();
 
@@ -416,13 +417,13 @@ retry:
 			}
 		}
 
-		if (memcmp(primaryFileBuf, mirrorFileBuf, primaryFileBytesRead) != 0)
+		if ((diff = memcmp(primaryFileBuf, mirrorFileBuf, primaryFileBytesRead)) != 0)
 		{
 			/* different contents */
 			ereport(NOTICE,
-					(errmsg("%s files \"%s\" and \"%s\" mismatch at blockno %u",
+					(errmsg("%s files \"%s\" and \"%s\" mismatch by %i at blockno %u",
 							get_relation_type_data(rentry->relam, rentry->relstorage, rentry->relkind).name,
-							primaryfilepath, mirrorfilepath, blockno)));
+							primaryfilepath, mirrorfilepath, diff, blockno)));
 			goto retry;
 		}