patch 8.1.0602: DirChanged is also triggered when directory didn't change

Problem:    DirChanged is also triggered when the directory didn't change.
            (Daniel Hahler)
Solution:   Compare the current with the new directory. (closes #3697)

src/testdir/test_autochdir.vim

@@ -8,11 +8,19 @@ func Test_set_filename()
   let cwd = getcwd()
   call test_autochdir()
   set acd
+
+  let s:li = []
+  autocmd DirChanged auto call add(s:li, "autocd")
+  autocmd DirChanged auto call add(s:li, expand("<afile>"))
+
   new
   w samples/Xtest
   call assert_equal("Xtest", expand('%'))
   call assert_equal("samples", substitute(getcwd(), '.*/\(\k*\)', '\1', ''))
+  call assert_equal(["autocd", getcwd()], s:li)
+
   bwipe!
+  au! DirChanged
   set noacd
   exe 'cd ' . cwd
   call delete('samples/Xtest')