Rerender a specific tag/unreleased in the changelog

[ruben-arts]

Describe the solution you'd like

I would like to use git-cliff in a GitHub action that runs after every merged PR. Updating the [unreleased] section. My initial instinct was that using the --prepend would have this behavior.

Describe alternatives you've considered

I could write some find replace code to manually do the replacement using the output of git-cliff.

Additional context

Example of the wokflow I'm looking for:

# CHANGELOG

## [1.2.3]
- change by @person in #1234

git merge feature/x
git-cliff -u --prepend changelog.md

# CHANGELOG

## [unreleased]
- feature x by @person in #1235

## [1.2.3]
- change by @person in #1234

git merge feature/y
git-cliff -u --prepend changelog.md

# CHANGELOG

## [unreleased]
- feature y by @person in #1236
- feature x by @person in #1235

## [1.2.3]
- change by @person in #1234

[welcome[bot]]

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[orhun]

Also reported in #544

[orhun]

FWIW we can use the markdown crate for parsing the changelog.