patch 8.0.0592: if a job writes to a buffer screen is not updated

Problem:    If a job writes to a buffer and the user is typing a command, the
            screen isn't updated. When a message is displayed the changed
            buffer may cause it to be cleared. (Ramel Eshed)
Solution:   Update the screen and then the command line if the screen didn't
            scroll. Avoid inserting screen lines, as it clears any message.
            Update the status line when the buffer changed.

src/vim.h

@@ -630,6 +630,8 @@ extern int (*dyn_libintl_putenv)(const char *envstring);
  * flags for update_screen()
  * The higher the value, the higher the priority
  */
+#define VALID_NO_UPDATE		 5  /* no new changes, keep the command line if
+				       possible */
 #define VALID			10  /* buffer not changed, or changes marked
 				       with b_mod_* */
 #define INVERTED		20  /* redisplay inverted part that changed */