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)

runtime/doc/eval.txt

@@ -10864,7 +10864,10 @@ win_getid([{win} [, {tab}]])				*win_getid()*
 
 win_gettype([{nr}])					*win_gettype()*
 		Return the type of the window:
+			"aucmdwin"	autocommand window. Temporary window
+					used to execute autocommands.
 			"popup"		popup window |popup|
+			"preview"	preview window |preview-window|
 			"command"	command-line window |cmdwin|
 			(empty)		normal window
 			"unknown"	window {nr} not found

src/evalwindow.c

@@ -858,13 +858,18 @@ f_win_gettype(typval_T *argvars, typval_T *rettv)
 	    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
 }

src/testdir/test_autocmd.vim

@@ -2579,4 +2579,26 @@ func Test_BufDelete_changebuf()
   close!
 endfunc
 
+" Test for the temporary internal window used to execute autocmds
+func Test_autocmd_window()
+  %bw!
+  edit one.txt
+  tabnew two.txt
+  let g:blist = []
+  augroup aucmd_win_test
+    au!
+    au BufEnter * call add(g:blist, [expand('<afile>'),
+          \ win_gettype(bufwinnr(expand('<afile>')))])
+  augroup END
+
+  doautoall BufEnter
+  call assert_equal([['one.txt', 'aucmdwin'], ['two.txt', '']], g:blist)
+
+  augroup aucmd_win_test
+    au!
+  augroup END
+  augroup! aucmd_win_test
+  %bw!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/testdir/test_preview.vim

@@ -25,6 +25,7 @@ func Test_window_preview()
   " Go to the preview window
   wincmd P
   call assert_equal(1, &previewwindow)
+  call assert_equal('preview', win_gettype())
 
   " Close preview window
   wincmd z

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    991,
 /**/
     990,
 /**/