patch 8.2.3671: restarting Insert mode in prompt buffer too often

Problem:    Restarting Insert mode in prompt buffer too often when a callback
            switches windows and comes back. (Sean Dewar)
Solution:   Do not set "restart_edit" when already in Insert mode.
diff --git a/src/window.c b/src/window.c
index d28962a..4fbf944 100644
--- a/src/window.c
+++ b/src/window.c
@@ -2268,8 +2268,9 @@
 	stop_insert_mode = FALSE;
 
     // When entering the prompt window restart Insert mode if we were in Insert
-    // mode when we left it.
-    restart_edit = win->w_buffer->b_prompt_insert;
+    // mode when we left it and not already in Insert mode.
+    if ((State & INSERT) == 0)
+	restart_edit = win->w_buffer->b_prompt_insert;
 }
 #endif