repart: Don't copy trailing padding when using --copy-from=

repart: Don't copy trailing padding when using --copy-from=

Currently, --copy-from= copies the paddings in between the source partitions,
as well as the trailing padding that is at the end of the source partition table.
It doesn't copy the leading padding at the start of the source partition table
though.

This seems inconsistent, and likely it was an oversight that the trailing padding
is copied.

Fix that, and add a test to ensure we don't regress.