fix(changelog): generating changelog after a pre-release

[woile]

Closes #357

Description

Trying to create an incremental changelog after a prerelease failed.
This happened because of the version regex when parsing the changelog.

Checklist

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

Expected behavior

You'll see the changelog created

Steps to Test This Pull Request

# step1 init everything, not exactly relevant to the bug
git init
cz init
git add .
git commit -m "chore: add config"
git tag 0.0.1

# step2 add a dummy feature and make a pre-release
git commit -m "feat: first feat" --allow-empty
cz bump --prerelease rc --changelog

# step3 add another feature and try to do incremental changelog
git commit -m "feat: second feat" --allow-empty
cz changelog --incremental

[codecov]

Codecov Report

Merging #390 (d82727c) into master (96265ce) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #390   +/-   ##
=======================================
  Coverage   97.90%   97.90%           
=======================================
  Files          39       39           
  Lines        1381     1381           
=======================================
  Hits         1352     1352           
  Misses         29       29           

Flag	Coverage Δ
unittests	97.90% <100.00%> (ø)

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

Impacted Files	Coverage Δ
commitizen/defaults.py	100.00% <100.00%> (ø)

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[Lee-W]

👍