patch 8.2.0261: some code not covered by tests

Problem:    Some code not covered by tests.
Solution:   Add test cases. (Yegappan Lakshmanan, closes #5645)
diff --git a/src/testdir/test_filechanged.vim b/src/testdir/test_filechanged.vim
index 6d17d95..6192b9e 100644
--- a/src/testdir/test_filechanged.vim
+++ b/src/testdir/test_filechanged.vim
@@ -146,3 +146,18 @@
   bwipe!
   call delete('Xchanged_d')
 endfunc
+
+" Test for editing a new buffer from a FileChangedShell autocmd
+func Test_FileChangedShell_newbuf()
+  call writefile(['one', 'two'], 'Xfile')
+  new Xfile
+  augroup testnewbuf
+    autocmd FileChangedShell * enew
+  augroup END
+  call writefile(['red'], 'Xfile')
+  call assert_fails('checktime', 'E811:')
+  au! testnewbuf
+  call delete('Xfile')
+endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab