dvc: safely return to saved tree in brancher

dvc/repo/brancher.py

@@ -42,13 +42,9 @@ def brancher(  # noqa: E302
         if all_tags:
             revs.extend(scm.list_tags())
 
-    # NOTE: it might be a good idea to wrap this loop in try/finally block
-    # to don't leave the tree on some unexpected branch after the
-    # `brancher()`, but this could cause problems on exception handling
-    # code which might expect the tree on which exception was raised to
-    # stay in place. This behavior is a subject to change.
-    for sha, names in group_by(scm.resolve_rev, revs).items():
-        self.tree = CleanTree(scm.get_tree(sha))
-        yield ", ".join(names)
-
-    self.tree = saved_tree
+    try:
+        for sha, names in group_by(scm.resolve_rev, revs).items():
+            self.tree = CleanTree(scm.get_tree(sha))
+            yield ", ".join(names)
+    finally:
+        self.tree = saved_tree