patch 9.0.1439: start Insert mode when accessing a hidden prompt buffer

Problem:    Start Insert mode when accessing a hidden prompt buffer.
Solution:   Call leaving_window() in aucmd_restbuf(). (Thorben Tröbst,
            closes #12148, closes #12147)

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1439,
 /**/
     1438,
 /**/