Add ability to re-generate the entire changelog

[MarcoIeni]

Motivations

Release-plz only appends the new changes to the changelog.
It would be nice if it could generate the whole changelog from scratch. This can be useful because:

• if you change the changelog configuration, you can apply the changes also to previous releases
• if you haven't used release-plz before, you can generate the changelog for previous versions

Solution

This should be a new release-plz subcommand. release-plz update should only append the changes for the new version.

TODO

Additional context

Suggested here

[intgr]

One can also call git cliff directly. By default it will generate the whole changelog.

[MarcoIeni]

yes, we just need to make sure to use the same release-plz config:

release-plz/crates/release_plz_core/src/changelog.rs

Line 225 in c5eed2f

fn default_changelog_body_config(release_link: Option<&str>) -> String {

I need to document this better