vim-patch:8.0.1190: unusable after opening new window in BufWritePre event

Problem:    Vim becomes unusable after opening new window in BufWritePre
            event.
Solution:   Call not_exiting(). (Martin Tournoij, closes vim/vim#2205)
            Also for "2q" when a help window is open.  Add a test.
2c33d7bb69

src/nvim/ex_docmd.c

@@ -6025,6 +6025,7 @@ static void ex_quit(exarg_T *eap)
     if (only_one_window() && (ONE_WINDOW || eap->addr_count == 0)) {
       getout(0);
     }
+    not_exiting();
     // close window; may free buffer
     win_close(wp, !buf_hide(wp->w_buffer) || eap->forceit);
   }
@@ -6382,6 +6383,7 @@ static void ex_exit(exarg_T *eap)
       // quit last window, exit Vim
       getout(0);
     }
+    not_exiting();
     // Quit current window, may free the buffer.
     win_close(curwin, !buf_hide(curwin->w_buffer));
   }

src/nvim/testdir/test_writefile.vim

@@ -31,3 +31,47 @@ func Test_writefile_fails_gently()
 
   call assert_fails('call writefile([], [])', 'E730:')
 endfunc
+
+func SetFlag(timer)
+  let g:flag = 1
+endfunc
+
+func Test_write_quit_split()
+  " Prevent exiting by splitting window on file write.
+  augroup testgroup
+    autocmd BufWritePre * split
+  augroup END
+  e! Xfile
+  call setline(1, 'nothing')
+  wq
+
+  if has('timers')
+    " timer will not run if "exiting" is still set
+    let g:flag = 0
+    call timer_start(1, 'SetFlag')
+    sleep 50m
+    call assert_equal(1, g:flag)
+    unlet g:flag
+  endif
+  au! testgroup
+  bwipe Xfile
+  call delete('Xfile')
+endfunc
+
+func Test_nowrite_quit_split()
+  " Prevent exiting by opening a help window.
+  e! Xfile
+  help
+  wincmd w
+  exe winnr() . 'q'
+
+  if has('timers')
+    " timer will not run if "exiting" is still set
+    let g:flag = 0
+    call timer_start(1, 'SetFlag')
+    sleep 50m
+    call assert_equal(1, g:flag)
+    unlet g:flag
+  endif
+  bwipe Xfile
+endfunc