vim-patch:8.1.1870: using :pedit from a help file sets help filetype

Problem:    Using :pedit from a help file sets the preview window to help
            filetype. (Wang Shidong)
Solution:   Do not set "keep_help_flag". (closes vim/vim#3536)
026587b35c

src/nvim/ex_docmd.c

@@ -8432,13 +8432,15 @@ static void ex_pedit(exarg_T *eap)
 {
   win_T       *curwin_save = curwin;
 
+  // Open the preview window or popup and make it the current window.
   g_do_tagpreview = p_pvh;
   prepare_tagpreview(true);
-  keep_help_flag = bt_help(curwin_save->w_buffer);
+
+  // Edit the file.
   do_exedit(eap, NULL);
-  keep_help_flag = FALSE;
+
   if (curwin != curwin_save && win_valid(curwin_save)) {
-    /* Return cursor to where we were */
+    // Return cursor to where we were
     validate_cursor();
     redraw_later(VALID);
     win_enter(curwin_save, true);

src/nvim/testdir/test_window_cmd.vim

@@ -144,6 +144,21 @@ func Test_window_preview()
   call assert_fails('wincmd P', 'E441:')
 endfunc
 
+func Test_window_preview_from_help()
+  filetype on
+  call writefile(['/* some C code */'], 'Xpreview.c')
+  help
+  pedit Xpreview.c
+  wincmd P
+  call assert_equal(1, &previewwindow)
+  call assert_equal('c', &filetype)
+  wincmd z
+
+  filetype off
+  close
+  call delete('Xpreview.c')
+endfunc
+
 func Test_window_exchange()
   e Xa
 
@@ -519,6 +534,7 @@ func Test_winrestcmd()
 endfunc
 
 function! Fun_RenewFile()
+  " Need to wait a bit for the timestamp to be older.
   sleep 2
   silent execute '!echo "1" > tmp.txt'
   sp
@@ -536,7 +552,6 @@ func Test_window_prevwin()
   call writefile(['2'], 'tmp.txt')
   new tmp.txt
   q
-  " Need to wait a bit for the timestamp to be older.
   call Fun_RenewFile()
   call assert_equal(2, winnr())
   wincmd p