cmdutil.changeset_printer: pass context into showpatch()
Before, we passed the node then subsequently performed a lookup on
repo.changelog. We already has the context available, so just pass it
in.
This does result in a small performance win. But I doubt it will show
up anywhere because diff[stat] calculation will dwarf the time spent
to create a changectx. Still, we should be creating fewer changectx
out of principle.