Merge branch 'pw/gc-during-rebase'

The sequencer machinery does not use the ref API and instead records names of certain objects it needs for its correct operation in temporary files, which makes these objects susceptible to loss by garbage collection. These temporary files have been added as starting points for reachability analysis to fix this. * pw/gc-during-rebase: prune: mark rebase autostash and orig-head as reachable