Add utilities to easily patch commit dates and authors

[elamperti]

Sometimes it's useful to alter the date of a commit (specially after squashing) or even fix its author. It would be useful to have a simple interface to do so, available through memorable aliases.

• For the date, it would be nice to have some kind of date picker.
• For the author, suggest at least the name+email used by git config. It would also be nice to have other authors from past commits as secondary options just in case.