Otherwise a file could be created out of the working copy directory.
This only works for untracked symlinks and sequentially "added" symlinks
and files. For "removed" and "modified" entries, the parent directories are
considered valid and fs::remove_file() will be called. This also doesn't
prevent race conditions caused by concurrent checkouts.
New create_parent_dirs() would be slightly slower than the original because
it traverses directories from the root whereas fs::create_dir_all() does that
from the leaf and exits when reached to a directory.