patch 8.1.0341: :argadd in empty buffer changes the buffer name

Problem:    :argadd in empty buffer changes the buffer name. (Pavol Juhas)
Solution:   Don't re-use the current buffer when not going to edit the file.
            (closes #3397)  Do re-use the current buffer for :next.

src/testdir/test_command_count.vim

@@ -158,7 +158,9 @@ endfunc
 func Test_command_count_4()
   %argd
   let bufnr = bufnr('$')
-  arga aa bb cc dd ee ff
+  next aa bb cc dd ee ff
+  call assert_equal(bufnr, bufnr('%'))
+
   3argu
   let args = []
   .,$-argdo call add(args, expand('%'))