patch 9.1.0040: issue with prompt buffer and hidden buffer

Problem:  Modifying a hidden buffer still interferes with prompt buffer
          mode changes.
Solution: Save and restore b_prompt_insert.
          (zeertzjq)

closes: #13875

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
Modifying hidden buffer still interferes with prompt buffer mode changes
diff --git a/src/structs.h b/src/structs.h
index 76f1433..5b88260 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -4358,7 +4358,6 @@
  */
 typedef struct
 {
-    buf_T	*save_curbuf;	    // saved curbuf
     int		use_aucmd_win_idx;  // index in aucmd_win[] if >= 0
     int		save_curwin_id;	    // ID of saved curwin
     int		new_curwin_id;	    // ID of new curwin
@@ -4367,6 +4366,9 @@
     char_u	*globaldir;	    // saved value of globaldir
     int		save_VIsual_active; // saved VIsual_active
     int		save_State;	    // saved State
+#ifdef FEAT_JOB_CHANNEL
+    int		save_prompt_insert; // saved b_prompt_insert
+#endif
 } aco_save_T;
 
 /*