superfluous query for ticket links for merged ticket id's

Hi,

if you merge ticket 2 into 1, all links are moved from ticket 2 to 1 [1].
If you later query ticket 1 for linked tickets, the link query limits by
the ticket id and RT::Ticket->Merged [2], which starts a query for
tickets that are merged into this ticket [3].

As ticket links are moved on merge, limit the RT::Ticket->_Links also by
merged ticket id's looks unnecessary for me.

Chris

[1] https://github.com/bestpractical/rt/blob/stable/lib/RT/Ticket.pm#L1849
[2] https://github.com/bestpractical/rt/blob/stable/lib/RT/Ticket.pm#L1751
[3] https://github.com/bestpractical/rt/blob/stable/lib/RT/Ticket.pm#L1751
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