rust-status: don't dispatch unknown file when traversing if not listing unknowns

This usually isn't a (functional) problem since we ignore the unknown files
anyway, but when specifically using `hg purge`, unknown files were iterated over
regardless of the option being true.
This is both more correct and more efficient.

Differential Revision: https://phab.mercurial-scm.org/D8517

rust/hg-core/src/dirstate/status.rs

@@ -371,9 +371,11 b" fn handle_traversed_entry<'a>("
                         .unwrap();
                 }
             } else {
-                files_sender
-                    .send(Ok((filename.to_owned(), Dispatch::Unknown)))
-                    .unwrap();
+                if options.list_unknown {
+                    files_sender
+                        .send(Ok((filename.to_owned(), Dispatch::Unknown)))
+                        .unwrap();
+                }
             }
         } else if ignore_fn(&filename) && options.list_ignored {
             files_sender