* trees: BaseTree.walk should handle errors like os.walk
* use onerror callback if exceptions should be logged or raised
* repo.ls: use onerror for tree.walk
* drop python 3.5 support
* dvc: remove fspath() use inside os.path calls
* dvc: replace dvc.compat.fspath with os.fspath
* tests: replace dvc.compat.fspath with os.fspath
* utils: relpath() should use fspath/str
* utils: os.readlink doesn't support path-like obj on Windows until 3.8
Current implementation doesn't work with directories, which results in
hard-to-debug bugs like #3561. The bug that made us walk through index
manually is now fixed in gitpython, so we can simply use `ls_files` from
now on. Added some tests to prevent this from happening in the future.
Fixes#3561
The `dvc/scm/__init__.py:scm_context` works with `Repo` objects and
is used mostly in `dvc/repo/...`, so it is better to put it in
`dvc/repo/scm_context.py`.
Fix#1769
Abstract access to filesystem and its implementation to access files and
particulary Stage objects in different git branches without `git
checkout` execution.