patch 9.1.1576: cannot easily trigger wildcard expansion

Problem:  cannot easily trigger wildcard expansion
Solution: Introduce wildtrigger() function
          (Girish Palya)

This PR introduces a new `wildtrigger()` function.

See `:h wildtrigger()`

`wildtrigger()` behaves like pressing the `wildchar,` but provides a
more refined and controlled completion experience:

- Suppresses beeps when no matches are found.
- Avoids displaying irrelevant completions (like full command lists)
  when the prefix is insufficient or doesn't match.
- Skips completion if the typeahead buffer has pending input or if a
  wildmenu is already active.
- Does not print "..." before completion.

This is an improvement on the `feedkeys()` based autocompletion script
given in #16759.

closes: #17806

Signed-off-by: Girish Palya <girishji@gmail.com>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/cmdexpand.c

@@ -238,6 +238,7 @@ nextwild(
     cmdline_info_T	*ccline = get_cmdline_info();
     int		i;
     char_u	*p;
+    int		from_wildtrigger_func = options & WILD_FUNC_TRIGGER;
 
     if (xp->xp_numfiles == -1)
     {
@@ -269,17 +270,22 @@ nextwild(
 	return FAIL;
     }
 
+    i = (int)(xp->xp_pattern - ccline->cmdbuff);
+    xp->xp_pattern_len = ccline->cmdpos - i;
+
+    // Skip showing matches if prefix is invalid during wildtrigger()
+    if (from_wildtrigger_func && xp->xp_context == EXPAND_COMMANDS
+	    && xp->xp_pattern_len == 0)
+	return FAIL;
+
     // If cmd_silent is set then don't show the dots, because redrawcmd() below
     // won't remove them.
-    if (!cmd_silent)
+    if (!cmd_silent && !from_wildtrigger_func)
     {
 	msg_puts("...");	    // show that we are busy
 	out_flush();
     }
 
-    i = (int)(xp->xp_pattern - ccline->cmdbuff);
-    xp->xp_pattern_len = ccline->cmdpos - i;
-
     if (type == WILD_NEXT || type == WILD_PREV
 	    || type == WILD_PAGEUP || type == WILD_PAGEDOWN)
     {