vim-patch:9.1.0821: 'findexpr' completion doesn't set v:fname to cmdline argument

Problem:  'findexpr' completion doesn't set v:fname to cmdline argument.
Solution: Set v:fname to the cmdline argument as-is (zeertzjq).

closes: vim/vim#15934

20e045f781

src/nvim/vvars.lua

@@ -50,6 +50,13 @@ M.vars = {
       can be used.
     ]=],
   },
+  cmdcomplete = {
+    type = 'boolean',
+    desc = [=[
+      When evaluating 'findexpr': if 'findexpr' is used for cmdline
+      completion the value is |v:true|, otherwise it is |v:false|.
+    ]=],
+  },
   collate = {
     type = 'string',
     desc = [=[