rrepair, merkle: apply techniques from r7273
rrepair, merkle: apply techniques from r7273

this reduces trivial RC costs (2nd merkle phase) by up to 50% when items have to be regenerated
tectronics/scalaris,tectronics/scalaris,digshock/scalaris,seyyed/scalaris,digshock/scalaris,digshock/scalaris,seyyed/scalaris,tectronics/scalaris,digshock/scalaris,digshock/scalaris,seyyed/scalaris,tectronics/scalaris,digshock/scalaris,seyyed/scalaris,seyyed/scalaris,tectronics/scalaris,tectronics/scalaris,tectronics/scalaris,seyyed/scalaris,seyyed/scalaris,digshock/scalaris