[TF2] Fix Diamondback getting revenge crits from earlier kills when switched onto

[HyperionCoding]

Description

Fixes the following bug:

If a Spy performs backstabs or saps buildings with a different secondary weapon equipped, the Crits are retroactively awarded as soon as the Spy switches to the Diamondback via a Resupply locker or by picking up a dropped Diamondback.

Demonstration by SquimJim

[seanmcgeehan]

I don't think this should be merged. Many weapons work similarly (eyelander when switching loadouts) and spy is very weak. IMO we should be fixing the bugs that make spy weaker.