Fix quadratic behavior in Repo._collect_stages() (iterative#3932)

dvc/repo/__init__.py

@@ -527,14 +527,13 @@ def _collect_stages(self):
 
         for root, dirs, files in self.tree.walk(self.root_dir):
             for file_name in filter(is_valid_filename, files):
-                path = os.path.join(root, file_name)
-                stages.extend(self.get_stages(path))
+                new_stages = self.get_stages(os.path.join(root, file_name))
+                stages.extend(new_stages)
                 outs.update(
                     out.fspath
-                    for stage in stages
-                    for out in (
-                        out for out in stage.outs if out.scheme == "local"
-                    )
+                    for stage in new_stages
+                    for out in stage.outs
+                    if out.scheme == "local"
                 )
             dirs[:] = [d for d in dirs if os.path.join(root, d) not in outs]
         return stages