working_copy: in mtime race case, don't mutate current state

There's a comment saying that mutating the file's current state
simplifies later logic, but I don't think that's true. It might have
been true in the past, when we had `FileType::Conflict`.

lib/src/working_copy.rs

@@ -825,11 +825,9 @@ impl TreeState {
                 }
                 // If the file's mtime was set at the same time as this state file's own mtime,
                 // then we don't know if the file was modified before or after this state file.
-                // We set the file's mtime to 0 to simplify later code.
-                if current_file_state.mtime >= self.own_mtime {
-                    current_file_state.mtime = MillisSinceEpoch(0);
-                }
-                if current_file_state != &new_file_state {
+                if current_file_state != &new_file_state
+                    || current_file_state.mtime >= self.own_mtime
+                {
                     let new_file_type = new_file_state.file_type.clone();
                     *current_file_state = new_file_state;
                     let current_tree_value = current_tree.path_value(&repo_path);