Merge branch 'rr/rebase-autostash'

* rr/rebase-autostash:
  rebase: implement --[no-]autostash and rebase.autostash
  rebase --merge: return control to caller, for housekeeping
  rebase -i: return control to caller, for housekeeping
  am: return control to caller, for housekeeping
  rebase: prepare to do generic housekeeping
  rebase -i: don't error out if $state_dir already exists
  am: tighten a conditional that checks for $dotest