terminal: Don't enter terminal mode from :normal

Ref: https://github.com/junegunn/fzf.vim/issues/8#issuecomment-139209765

src/nvim/edit.c

@@ -254,7 +254,16 @@ edit (
 )
 {
   if (curbuf->terminal) {
-    terminal_enter();
+    if (ex_normal_busy) {
+      // don't enter terminal mode from `ex_normal`, which can result in all
+      // kinds of havoc(such as terminal mode recursiveness). Instead, set a
+      // flag that allow us to force-set the value of `restart_edit` before
+      // `ex_normal` returns
+      restart_edit = 'i';
+      force_restart_edit = true;
+    } else {
+      terminal_enter();
+    }
     return false;
   }
 

src/nvim/ex_docmd.c

@@ -7624,6 +7624,10 @@ void update_topline_cursor(void)
  */
 static void ex_normal(exarg_T *eap)
 {
+  if (curbuf->terminal) {
+    EMSG("Can't re-enter normal mode from terminal mode");
+    return;
+  }
   int save_msg_scroll = msg_scroll;
   int save_restart_edit = restart_edit;
   int save_msg_didout = msg_didout;
@@ -7715,7 +7719,14 @@ static void ex_normal(exarg_T *eap)
 
   --ex_normal_busy;
   msg_scroll = save_msg_scroll;
-  restart_edit = save_restart_edit;
+  if (force_restart_edit) {
+    force_restart_edit = false;
+  } else {
+    // some function called was aware of ex_normal and decided to override the
+    // value of restart_edit anyway. So far only used in terminal mode(see
+    // terminal_enter() in edit.c)
+    restart_edit = save_restart_edit;
+  }
   p_im = save_insertmode;
   finish_op = save_finish_op;
   opcount = save_opcount;

src/nvim/globals.h

@@ -823,6 +823,8 @@ EXTERN int no_u_sync INIT(= 0);         /* Don't call u_sync() */
 EXTERN int u_sync_once INIT(= 0);       /* Call u_sync() once when evaluating
                                            an expression. */
 
+EXTERN bool force_restart_edit INIT(= false);  // force restart_edit after
+                                               // ex_normal returns
 EXTERN int restart_edit INIT(= 0);      /* call edit when next cmd finished */
 EXTERN int arrow_used;                  /* Normally FALSE, set to TRUE after
                                          * hitting cursor key in insert mode.