Merge branch 'pw/rebase-keep-base-fixes' into pw/rebase-reflog-fixes

* pw/rebase-keep-base-fixes:
  rebase --keep-base: imply --no-fork-point
  rebase --keep-base: imply --reapply-cherry-picks
  rebase: factor out branch_base calculation
  rebase: rename merge_base to branch_base
  rebase: store orig_head as a commit
  rebase: be stricter when reading state files containing oids
  t3416: set $EDITOR in subshell
  t3416: tighten two tests