Add rake tasks to merge and create Changelog entries

[marcandre]

Fixes rubocop/rubocop#8697

After yet another Changelog merge issue, I decided to write some code to avoid it.

This PR adds a rake task to merge entries in CHANGELOG/ according to the prefix of their path: "new" (feature), (bug) "fix" or "change". It merges those with any existing entries in the unreleased section of the Changelog, so it can be done at any moment, is backward compatible, and nobody is forced to use it (except before cutting a release, and I've added a check for that 😅).

Other rake task cl:new, cl:fix, cl:change create an entry file (default use the last commit's message & git config).

For a contributor, the workflow looks like this:

• make a contribution, test it, etc.
• git commit (enter a good commit message, optionally starting with [Fixes #xxx])
• bundle exec rake cl:fix
• modify file if desired
• git commit --amend
• no worry about any changelog conflict / forgetting to move it to the right spot if a release is made before it is merged

@bbatsov any objection that I use this in rubocop-ast? May I adapt it for rubocop too?

[bbatsov]

@bbatsov any objection that I use this in rubocop-ast?

No objections.

May I adapt it for rubocop too?

Sure. Might be good to use it for a while in rubocop-ast and refine the workflow (if needed) first, though.