repart: Don't get old grain size from fdisk for --copy-from=

repart: Don't get old grain size from fdisk for --copy-from=

This is a little bit confusing, but grain size is not actually stored in the gpt
metadata. Rather, fdisk's `get_grain_size()` returns an autodiscovered "optimal io
size" value as grain size. This might not actually be the grain size that the
disk we're copying is using.

Since we're setting the padding of the copied partitions using that value from
fdisk, we're rounding the new paddings by fdisk's optimal grain size, which is
usually 1MiB (a lot more then the default 4KiB that we're using otherwise).

Set the grain size here to 1 byte instead, ensuring that the min/max padding set
is exactly the padding that was present before.

Also add a test to confirm the behavior is fixed: The test calls --copy-from= on
an existing disk with 4MiB grain size, and because we pass --grain-size=512, now
no rounding should happen and the paddings should be transferred to exactly the
same size.