fix(changelog): skip non-compliant commit subjects when building changelog

[woile]

This should fix duplicate entries in the changelog

Closes #510

Description

Checklist

• Add test cases to all the changes you introduce
• Run ./scripts/format and ./scripts/test locally to ensure this change passes linter check and test
• Test the changes on the local machine manually
• Update the documentation for the changes

Expected behavior

Only entries that have a subject compliant with conventional commits should be included.

Steps to Test This Pull Request

Run on our own commitizen:

cz ch --dry-run 2.20.5..v2.21.0

Before it was including duplicated entries, now only good ones.

Additional context

[codecov]

Codecov Report

Merging #536 (0307360) into master (a11f203) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #536   +/-   ##
=======================================
  Coverage   98.00%   98.00%           
=======================================
  Files          39       39           
  Lines        1551     1552    +1     
=======================================
+ Hits         1520     1521    +1     
  Misses         31       31           

Flag	Coverage Δ
unittests	98.00% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
commitizen/__version__.py	100.00% <100.00%> (ø)
...en/cz/conventional_commits/conventional_commits.py	100.00% <100.00%> (ø)

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