Fix nested commands in interactive mode triggering prompt2 (issue #19)

Save and restore cmd_mode across interpolation parsing so that
luaX_readcommandcont resumes in the correct mode after a nested
backtick command inside ${...}.

llex.c

@@ -512,6 +512,7 @@ static int read_command_body (LexState *ls, SemInfo *seminfo) {
           /* store fragment so far */
           seminfo->ts = luaX_newstring(ls, luaZ_buffer(ls->buff),
                                            luaZ_bufflen(ls->buff));
+          ls->saved_cmd_mode = ls->cmd_mode;  /* save for readcommandcont */
           return interactive ? TK_INTERACTIVE_INTERP : TK_COMMAND_INTERP;
         }
         else {
@@ -571,6 +572,7 @@ static int read_command (LexState *ls, SemInfo *seminfo) {
 ** consumed the '}' that closes an interpolated ${expr}.
 */
 int luaX_readcommandcont (LexState *ls) {
+  ls->cmd_mode = ls->saved_cmd_mode;  /* restore mode from before interpolation */
   int tk = read_command_body(ls, &ls->t.seminfo);
   ls->t.token = tk;
   return tk;