Copy the client into the new cmdq in source-file so commands that work

on it (such as new-session) can work. Fixes issue reported by oss-adv at
users dot sf dot net.

cmd-source-file.c

@@ -49,6 +49,7 @@ cmd_source_file_exec(struct cmd *self, struct cmd_q *cmdq)
 	char		*cause;
 
 	cmdq1 = cmdq_new(NULL);
+	cmdq1->client = cmdq->client;
 	cmdq1->emptyfn = cmd_source_file_done;
 	cmdq1->data = cmdq;