.arcconfig

tweakdefaults: make commands.update.check be `noconflict`

This is the second-safest option we have to offer in `updatecheck`,
with `abort` being the safest and `linear` being the default. At the
sprint we discussed how much `none` and `linear` make us all
uncomfortable, and how we'd like to move the default behavior if we can.

I'm not sure we can get away with actually changing the out of the box
default behavior, but we can at *least* do this.

Differential Revision: https://phab.mercurial-scm.org/D1062

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