"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