During a snapshot, when a file changed and not its parent directories,
the parent directories weren't added to the layer. This is inconsistent
with Docker's behavior which always add parent directories to the layer.
In some edge-cases, it could lead to problems with docker considering
that parent directories where owned by root in forthcoming layers
although they shouldn't (see GoogleContainerTools#1163).

Also, Docker seems to be POSIX compliant regarding the name of
directories in the archive, which always have a slash appended. This
commit also fixes this.

Fixes GoogleContainerTools#1163

This refactoring reversed the order of the "ParentDirectories" function: GoogleContainerTools@ffc372a#diff-d36eb675aa49a7b471e3a2be77005b18R465

As a side-effect, parent directories weren't added in lexicographical
order, which broke some tests. We now ensure in unit test that the order
of the ParentDirectories function is stable.