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