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 ${...}.

testes/lush/interactive.lua

@@ -86,6 +86,14 @@ do
   assert(run_cmd() == 0)
 end
 
+-- ===== NESTED BACKTICK IN INTERPOLATION (issue #19) =====
+
+do
+  local result = `echo hello`
+!echo ${result.stdout}
+  assert(_.code == 0)
+end
+
 -- ===== EMPTY COMMAND (just whitespace after !) =====
 
 do