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

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

"git rebase --keep-base" used to discard the commits that are
already cherry-picked to the upstream, even when "keep-base" meant
that the base, on top of which the history is being rebuilt, does
not yet include these cherry-picked commits.  The --keep-base
option now implies --reapply-cherry-picks and --no-fork-point
options.

* 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