repart: Don't descend into directories assigned to other partitions

repart: Don't descend into directories assigned to other partitions

Let's say we have the following repart definitions files root.conf
and home.conf:

```
[Partition]
Type=root
CopyFiles=/
```

```
[Partition]
Type=home
CopyFiles=/home
```

Currently, we'd end up copying /home to both the root partition and
the home partition. To prevent this from happening, let's adopt a
new policy when copying files for a partition: We won't copy any
files/directories that appear in the CopyFiles= list of another
partition, unless that directory explicitly appears in our own
CopyFiles= list.

This way, we prevent copying /home twice into the root and home
partition, but should a user really want that behavior, they can
have it by adding /home to the CopyFIles= list of the root partition
as well.