patch 8.2.3174: Vim9: "legacy undo" finds "undo" variable

Problem:    Vim9: "legacy undo" finds "undo" variable.
Solution:   Do not pass lookup function to find_ex_command(). (closes #8563)
diff --git a/src/testdir/test_vim9_assign.vim b/src/testdir/test_vim9_assign.vim
index 3599d6c..a33038c 100644
--- a/src/testdir/test_vim9_assign.vim
+++ b/src/testdir/test_vim9_assign.vim
@@ -1632,6 +1632,13 @@
     let s:legvar = 'one'
   END
   CheckScriptFailure(lines, 'E476:', 1)
+
+  edit! Xfile
+  lines =<< trim END
+      var edit: bool
+      legacy edit
+  END
+  CheckDefAndScriptSuccess(lines)
 enddef
 
 def Test_var_type_check()
diff --git a/src/version.c b/src/version.c
index f7e2c17..e1421dd 100644
--- a/src/version.c
+++ b/src/version.c
@@ -756,6 +756,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    3174,
+/**/
     3173,
 /**/
     3172,
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 956ce44..9ae11a5 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -9510,7 +9510,8 @@
 		}
 	    }
 	}
-	p = find_ex_command(&ea, NULL, starts_with_colon
+	p = find_ex_command(&ea, NULL,
+		starts_with_colon || (local_cmdmod.cmod_flags & CMOD_LEGACY)
 						  ? NULL : item_exists, &cctx);
 
 	if (p == NULL)