[Unison-hackers] move instead of copy

[Trevor Jim]

As I recall, the reason for the copy is that sometimes move is not
atomic.

For example, a move from one hard disk to another is actually a copy and
a delete.  The copy could fail in the middle.

It is hard to tell how a move will be implemented from just the path of
source and destination.

Using copy+delete instead of a move is therefore a safer approach, since
Unison knows that copies can fail and takes this into account so that
you do not lose data.

Something to keep in mind...

-Trevor