Result inconsistency for `__call__`, `is_ignored_file` and `is_ignored_dir`

[karajan1001]

Bug Report

Please provide information about your setup

Output of dvc version:

$ dvc version

master

Additional Information (if any):

If applicable, please also provide a --verbose output of the command, eg: dvc add --verbose.

Now there are several interfaces for the dvcignore.
In is_ignored_dir we ignored all paths outside the repo.
https://github.com/iterative/dvc/blob/35dd1fda9cfb4b645ae431f4621f2d1853d4fb7c/dvc/ignore.py#L270-L272
In a direct call __call__ all outside paths are included.
https://github.com/iterative/dvc/blob/35dd1fda9cfb4b645ae431f4621f2d1853d4fb7c/dvc/ignore.py#L241-L244
And in is_ignored_files there is a bug that makes us skipped "outside the repo" check and return False forever.
https://github.com/iterative/dvc/blob/35dd1fda9cfb4b645ae431f4621f2d1853d4fb7c/dvc/ignore.py#L275-L278

They should be unified. So in what condition do we need to mask files outside the repo? And should we put this logic in dvcignore?