vim-patch:8.2.1967: the session file does not restore the alternate file

Problem:    The session file does not restore the alternate file.
Solution:   Add ":balt".  Works like ":badd" and also sets the buffer as the
            alternate file.  Use it in the session file. (closes vim/vim#7269,
            closes vim/vim#6714)
59d8e56e04

Include minimal test_buffer.vim from patch 8.2.0243 for Test_balt().
Add entry for :balt to runtime/doc/index.txt from vim/vim#7819.

src/nvim/ex_cmds.lua

@@ -175,6 +175,12 @@ module.cmds = {
     addr_type='ADDR_NONE',
     func='ex_edit',
   },
+  {
+    command='balt',
+    flags=bit.bor(NEEDARG, FILE1, CMDARG, TRLBAR, CMDWIN),
+    addr_type='ADDR_NONE',
+    func='ex_edit',
+  },
   {
     command='bdelete',
     flags=bit.bor(BANG, RANGE, BUFNAME, COUNT, EXTRA, TRLBAR),