read relative exclude paths from files within backup source

I current back up my home machine with `rsync` and make use of its `-F` option.

```
-F The -F option is a shorthand for adding two --filter rules to your command. The first time it is used is a short‐
hand for this rule:
--filter='dir-merge /.rsync-filter'[/+]
This tells rsync to look for per-directory .rsync-filter files that have been sprinkled through the hierarchy and
use their rules to filter the files in the transfer. If -F is repeated, it is a shorthand for this rule:
--filter='exclude .rsync-filter'
```