fix(changelog): do not change the tag date if tag already exists

[orhun]

Description

This PR fixes the issue where the tag date is always set to now() although it should be the date of the existing tag when --tag is used.

Motivation and Context

see #796

How Has This Been Tested?

Fixtures / 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]

Codecov Report

❌ Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 40.21%. Comparing base (c2db791) to head (c1a6fd7).
⚠️ Report is 196 commits behind head on main.

Files with missing lines	Patch %	Lines
git-cliff/src/lib.rs	0.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #861      +/-   ##
==========================================
- Coverage   40.31%   40.21%   -0.09%     
==========================================
  Files          21       21              
  Lines        1665     1669       +4     
==========================================
  Hits          671      671              
- Misses        994      998       +4     

Flag	Coverage Δ
unit-tests	40.21% <0.00%> (-0.09%)	⬇️

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