patch 8.2.0991: cannot get window type for autocmd and preview window

Problem:    Cannot get window type for autocmd and preview window.
Solution:   Add types to win_gettype(). (Yegappan Lakshmanan, closes #6277)
diff --git a/src/evalwindow.c b/src/evalwindow.c
index 0db45bc..585ff72 100644
--- a/src/evalwindow.c
+++ b/src/evalwindow.c
@@ -858,13 +858,18 @@
 	    return;
 	}
     }
+    if (wp == aucmd_win)
+	rettv->vval.v_string = vim_strsave((char_u *)"aucmdwin");
+#if defined(FEAT_QUICKFIX)
+    else if (wp->w_p_pvw)
+	rettv->vval.v_string = vim_strsave((char_u *)"preview");
+#endif
 #ifdef FEAT_PROP_POPUP
-    if (WIN_IS_POPUP(wp))
+    else if (WIN_IS_POPUP(wp))
 	rettv->vval.v_string = vim_strsave((char_u *)"popup");
-    else
 #endif
 #ifdef FEAT_CMDWIN
-    if (wp == curwin && cmdwin_type != 0)
+    else if (wp == curwin && cmdwin_type != 0)
 	rettv->vval.v_string = vim_strsave((char_u *)"command");
 #endif
 }