fix(changelog): correctly set the tag message for the latest release

[orhun]

Description

Now we support the following use case:

$ git cliff --unreleased --tag 1.2.3 --tag-message "Release 1.2.3"

Motivation and Context

closes #836

How Has This Been Tested?

Locally.

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (no code change)
• Refactor (refactoring production code)
• Other

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.
• I have formatted the code with rustfmt.
• I checked the lints with clippy.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.

Project coverage is 39.92%. Comparing base (fe37d14) to head (c0a02a3).

Files with missing lines	Patch %	Lines
git-cliff/src/lib.rs	0.00%	5 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #854      +/-   ##
==========================================
- Coverage   39.94%   39.92%   -0.02%     
==========================================
  Files          20       20              
  Lines        1660     1661       +1     
==========================================
  Hits          663      663              
- Misses        997      998       +1     

Flag	Coverage Δ
unit-tests	39.92% <0.00%> (-0.02%)	⬇️

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