Merge branch 'pw/use-in-process-checkout-in-rebase'

Merge branch 'pw/use-in-process-checkout-in-rebase'

Use an internal call to reset_head() helper function instead of
spawning "git checkout" in "rebase", and update code paths that are
involved in the change.

* pw/use-in-process-checkout-in-rebase:
  rebase -m: don't fork git checkout
  rebase --apply: set ORIG_HEAD correctly
  rebase --apply: fix reflog
  reset_head(): take struct rebase_head_opts
  rebase: cleanup reset_head() calls
  create_autostash(): remove unneeded parameter
  reset_head(): make default_reflog_action optional
  reset_head(): factor out ref updates
  reset_head(): remove action parameter
  rebase --apply: don't run post-checkout hook if there is an error
  rebase: do not remove untracked files on checkout
  rebase: pass correct arguments to post-checkout hook
  t5403: refactor rebase post-checkout hook tests
  rebase: factor out checkout for up to date branch