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/cmdexpand_defs.h

@@ -107,6 +107,7 @@ enum {
   EXPAND_KEYMAP,
   EXPAND_DIRS_IN_CDPATH,
   EXPAND_SHELLCMDLINE,
+  EXPAND_FINDEXPR,
   EXPAND_CHECKHEALTH,
   EXPAND_LUA,
 };