[Feature Request] checkservices: pass `-3` option through to pacdiff

[malsyned]

pacdiff accepts -3 / --threeway to look for the previous upstream package version of an etc file and do a 3-way compare / merge. I would love to be able to ask checkservices to enable that option to pacdiff by passing -3 to it.

[Antiz96]

Hi,

Thanks for the suggestion.
Should be covered in #100 :)