fix(1694): changelog_merge_prerelease not working on cz bump

[zimmermannJakob]

Description

This PR solves issue #1694. This PR is still in draft, as I want to check whether the changes make sense conceptually and manual tests are still pending.

Checklist

• I have read the contributing guidelines

Code Changes

• Add test cases to all the changes you introduce
• Run poetry all locally to ensure this change passes linter check and tests (Some error out on mac. I will investigate if they fail in ci)
• Manually test the changes:
  • Verify the feature/bug fix works as expected in real-world scenarios
  • Test edge cases and error conditions
  • Ensure backward compatibility is maintained
  • Document any manual testing steps performed
• Update the documentation for the changes

Documentation Changes

• Run poetry doc locally to ensure the documentation pages renders correctly
• Check and fix any broken links (internal or external) in the documentation

When running poetry doc, any broken internal documentation links will be reported in the console output like this:

INFO    -  Doc file 'config.md' contains a link 'commands/bump.md#-post_bump_hooks', but the doc 'commands/bump.md' does not contain an anchor '#-post_bump_hooks'.

Expected Behavior

See issue #1694.

Steps to Test This Pull Request

1. build the changes on the branch. poetry build.
2. Take the resulting .wheel file from dist/
3. Install the file in a separate python environment:

• python3 -m venv .venv && source .venv/bin/activate
• verify env: which pip
• pip install commitizen-4.10.0-py3-none-any.whl

4. Go through the steps in the issue description and try reproducing the bug.

Additional Context

Known issue: When the changelog only contains pre-releases, the old behavior can still be observed.