patch 8.2.0050: after deleting a file mark it is still in viminfo

Problem:    After deleting a file mark it is still in viminfo.
Solution:   When a file mark was deleted more recently than the mark in the
            merged viminfo file was updated, do not store the mark. (Pavol
            Juhas, closes #5401, closes #1339)

src/testdir/test_marks.vim

@@ -144,6 +144,11 @@ func Test_delmarks()
   " Deleting an already deleted mark should not fail.
   delmarks x
 
+  " getpos() should return all zeros after deleting a filemark.
+  norm mA
+  delmarks A
+  call assert_equal([0, 0, 0, 0], getpos("'A"))
+
   " Test deleting a range of marks.
   norm ma
   norm mb