patch 8.2.3629: command completion in cmdline window uses global commands

Problem:    Command completion in cmdline window uses global user commands,
            not local commands for the window where it was opened from.
Solution:   Use local commands. (closes #9168)

src/ex_getln.c

@@ -4485,6 +4485,15 @@ open_cmdwin(void)
 
     return cmdwin_result;
 }
+
+/*
+ * Return TRUE if in the cmdwin, not editing the command line.
+ */
+    int
+is_in_cmdwin(void)
+{
+    return cmdwin_type != 0 && get_cmdline_type() == NUL;
+}
 #endif // FEAT_CMDWIN
 
 /*