vim-patch:9.2.0494: User commands cannot handle single args with spaces (#39854)

Problem:  User commands cannot handle single args with spaces
Solution: Add the -nargs=_ attribute (Maxim Kim)

-nargs=_ allow user commands to have a single argument with spaces.

For example given the following Test command and TestComplete function:

```
vim9script
def TestComplete(A: string, _: string, _: number): list<string>
    var all = ["qqqq", "aaaa", "qq aa"]
    return all->matchfuzzy(A)
enddef
command! -nargs=_ -complete=customlist,TestComplete Test echo <q-args>
```

`:Test q a<tab>` should successfully complete `qq aa`

fixes:  vim/vim#20102
closes: vim/vim#20189

https://github.com/vim/vim/commit/f0e874a129a702457a383017b576eef1553f95d8

Co-authored-by: Maxim Kim <habamax@gmail.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/src/nvim/lua/executor.c b/src/nvim/lua/executor.c
index b785ed0c64..b6847f0492 100644
--- a/src/nvim/lua/executor.c
+++ b/src/nvim/lua/executor.c
@@ -2395,7 +2395,9 @@ int nlua_do_ucmd(ucmd_T *cmd, exarg_T *eap, bool preview)
   char nargs[2];
   if (cmd->uc_argt & EX_EXTRA) {
     if (cmd->uc_argt & EX_NOSPC) {
-      if (cmd->uc_argt & EX_NEEDARG) {
+      if (cmd->uc_argt & EX_ARGSPACE) {
+        nargs[0] = '_';
+      } else if (cmd->uc_argt & EX_NEEDARG) {
         nargs[0] = '1';
       } else {
         nargs[0] = '?';