Fix off by one sector error in GParted internal block copy (#742920)

GParted's internal block copy has an off by one sector bug when the
source is before the destination; and the copy is performed backwards
from high block to low block.  It is as though the source and
destination partitions were both one sector earlier on the disk.

In ASCII art it it looks like this:

Initial layout:      x<--SRC--><--DST-->
Actually wrote:               x<--SRC--
Should have written:           <--SRC-->

Affects moving partitions too.

This bug has existed since commit:

    bd9e16f2
    Thread the internal copy algorithm (#685740)

Effectively the last sector of the partition is missed and one sector
before the start of the partition is corrupted.  Most of the time file
systems don't get around to using the very last sector so didn't notice.

Bug 742920 - GParted internal copy is overstepping partition boundaries