vim-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 vim/vim#5401, closes vim/vim#1339)
https://github.com/vim/vim/commit/8cd6cd8087ccf08e4303dbf5f732fc4b82b917e1

Neovim's ShaDa is incompatible with Vim's "viminfo"
so "viminfo" tests fail.

N/A patches for version.c:

vim-patch:8.1.1731: command line history not read from viminfo on startup

Problem:    Command line history not read from viminfo on startup.
Solution:   Get history length after initializing it.
https://github.com/vim/vim/commit/26b654a5df9414e43734eb4c956b67c331d70a50

vim-patch:8.1.1764: ":browse oldfiles" is not tested

Problem:    ":browse oldfiles" is not tested.
Solution:   Add a test.
https://github.com/vim/vim/commit/5328cb8986d2620f45b41acf28778f8ce2f8cac1

vim-patch:8.1.2111: viminfo file not sufficiently tested

Problem:    Viminfo file not sufficiently tested.
Solution:   Add more tests. (Yegappan Lakshmanan, closes vim/vim#5009)
https://github.com/vim/vim/commit/2a8d3b8997d4fe94bc9c02ae04e873eab2f13b09

vim-patch:8.1.2126: viminfo not sufficiently tested

Problem:    Viminfo not sufficiently tested.
Solution:   Add more test cases.  Clean up comments. (Yegappan Lakshmanan,
            closes vim/vim#5032)
https://github.com/vim/vim/commit/6bd1d7706766a7899904163e8fd55ea117fb1953

src/nvim/mark.c

@@ -777,6 +777,7 @@ void ex_delmarks(exarg_T *eap)
               n = i - 'A';
             }
             namedfm[n].fmark.mark.lnum = 0;
+            namedfm[n].fmark.fnum = 0;
             XFREE_CLEAR(namedfm[n].fname);
           }
         }

src/nvim/testdir/test_marks.vim

@@ -171,6 +171,11 @@ 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