fix(bump): add changelog file into stage when running `cz bump --changelog

[Lee-W]

Types of changes

• Bug fix
• New feature
• Refactoring
• Breaking change (any change that would cause existing functionality to not work as expected)
• Documentation update
• Other (please describe)

Description

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
• Update the documentation for the changes

Steps to Test This Pull Request

mkdir bump_test
cd bump_test
git init
cz init
touch some_file.txt
git add .
git commit -m "feat: added some_file"
cz bump --changelog

Expected behavior

The CHANGELOG.md is added to git repo.

Related Issue

#221

Additional context

[codecov]

Codecov Report

Merging #228 into master will decrease coverage by 0.05%.
The diff coverage is 84.21%.

@@            Coverage Diff             @@
##           master     #228      +/-   ##
==========================================
- Coverage   95.64%   95.59%   -0.06%     
==========================================
  Files          34       34              
  Lines         942      954      +12     
==========================================
+ Hits          901      912      +11     
- Misses         41       42       +1     

Flag	Coverage Δ
#unittests	95.59% <84.21%> (-0.06%)	⬇️

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

Impacted Files	Coverage Δ
commitizen/commands/bump.py	90.58% <50.00%> (-0.88%)	⬇️
commitizen/__version__.py	100.00% <100.00%> (ø)
commitizen/commands/changelog.py	97.29% <100.00%> (+0.07%)	⬆️
commitizen/commands/commit.py	96.61% <100.00%> (+0.11%)	⬆️
commitizen/config/__init__.py	97.67% <100.00%> (ø)
commitizen/git.py	95.45% <100.00%> (+0.27%)	⬆️

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