Merge branch 'ra/rebase-i-more-options'

Merge branch 'ra/rebase-i-more-options'

"git rebase -i" learned a few options that are known by "git
rebase" proper.

* ra/rebase-i-more-options:
  rebase -i: finishing touches to --reset-author-date
  rebase: add --reset-author-date
  rebase -i: support --ignore-date
  sequencer: rename amend_author to author_to_rename
  rebase -i: support --committer-date-is-author-date
  sequencer: allow callers of read_author_script() to ignore fields
  rebase -i: add --ignore-whitespace flag