bug#27530: patch to cut and copy secondary

[Tak Kunihiro]

>> The function exchanges point and mark.
>> When there is region, this also creates secondary.
>> When there is secondary, this converts secondary to primary in advance.
> 
> OK. So if I've understood correctly, the user can then use C-y to yank
> the primary (which was the secondary before)?

The function `mouse-exchange-point-and-mark-secondary' has nothing to
do with kill-ring.

I suppose you meant if the user can C-w to kill the primary (which was
the secondary before). Yes.