Remove dvcignore reset in repo._reset (#4371)

* Remove dvcignore reset in `repo._reset`

Repo `add`,`checkout`,etc will not change `.dvcignore`, and can remove dvcignore reset after these operations.

* Reset dvcignore manually after a subrepo created.

1. subrepo created during session open time.

dvc/repo/__init__.py

@@ -615,4 +615,3 @@ class Repo:
         self.__dict__.pop("graph", None)
         self.__dict__.pop("stages", None)
         self.__dict__.pop("pipelines", None)
-        self.tree.__dict__.pop("dvcignore", None)

tests/func/test_tree.py

@@ -258,6 +258,7 @@ def test_walk_dont_ignore_subrepos(tmp_dir, scm, dvc):
     scm.commit("Add subrepo")
 
     dvc_tree = dvc.tree
+    dvc_tree.__dict__.pop("dvcignore")
     scm_tree = scm.get_tree("HEAD", use_dvcignore=True)
     path = os.fspath(tmp_dir)
     get_dirs = itemgetter(1)