index.html should be in the upload and cache folders in order to avoid direct access

I wrote a more explanatory comment regarding the cache directory
and why it is ignored.

With SugarCRM 6.4 the `cache/upload` directory moved to `upload` and
`upload_backup`. These directories can safely be ignored.

Additionally the `cache` directory is now ignored completely because it
leads to the behaviour to use the git repository as a kind of backup
tool if several parts of it are included. Furthermore the directory
structure beneath `cache` is victim of enduring changes by the
developers.

I did also some code cleanup regarding the path names e.g. adding a `/`
before most file and directory names.

This reverts commit 8e27618144.

be absolute because there may be some files with the same names
deeper in the path structure.