dvcignore: use correct root .dvcignore for subrepos (#3895)

* ignore: dvcignore root is distinct from tree_root

* for subrepos, dvcignore root should be the subrepo's .dvcignore, not
  host repo's .dvcignore

* tests: add test for subrepo dvcignore behavior

dvc/ignore.py

@@ -92,13 +92,14 @@ class DvcIgnoreRepo(DvcIgnore):
 
 
 class DvcIgnoreFilter:
-    def __init__(self, tree):
+    def __init__(self, tree, root_dir):
         self.tree = tree
+        self.root_dir = root_dir
         self.ignores = {
             DvcIgnoreDirs([".git", ".hg", ".dvc"]),
             DvcIgnoreRepo(),
         }
-        for root, dirs, files in self.tree.walk(self.tree.tree_root):
+        for root, dirs, files in self.tree.walk(self.root_dir):
             self._update(root)
             dirs[:], files[:] = self(root, dirs, files)
 
@@ -124,7 +125,7 @@ class CleanTree(BaseTree):
 
     @cached_property
     def dvcignore(self):
-        return DvcIgnoreFilter(self.tree)
+        return DvcIgnoreFilter(self.tree, self.tree_root)
 
     @property
     def tree_root(self):

tests/func/test_ignore.py

@@ -11,6 +11,7 @@ from dvc.ignore import (
     DvcIgnoreRepo,
 )
 from dvc.remote import LocalRemote
+from dvc.repo import Repo
 from dvc.scm.tree import WorkingTree
 from dvc.utils import relpath
 from dvc.utils.fs import get_mtime_and_size
@@ -151,3 +152,20 @@ def test_ignore_external(tmp_dir, scm, dvc, tmp_path_factory):
     remote = LocalRemote(dvc, {})
     result = {relpath(f, ext_dir) for f in remote.walk_files(ext_dir)}
     assert result == {"y.backup", "tmp"}
+
+
+def test_ignore_subrepo(tmp_dir, scm, dvc):
+    tmp_dir.gen({".dvcignore": "foo", "subdir": {"foo": "foo"}})
+    scm.add([".dvcignore"])
+    scm.commit("init parent dvcignore")
+
+    subrepo_dir = tmp_dir / "subdir"
+    assert not dvc.tree.exists(subrepo_dir / "foo")
+
+    with subrepo_dir.chdir():
+        subrepo = Repo.init(subdir=True)
+        scm.add(str(subrepo_dir / "foo"))
+        scm.commit("subrepo init")
+
+    for _ in subrepo.brancher(all_commits=True):
+        assert subrepo.tree.exists(subrepo_dir / "foo")