fix(git): fix get_commits deliminator

[Lee-W]

Description

• fix(git): fix get_commits deliminator
  • when we merge a pull request, it generates a commit without breakline

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

Expected behavior

Changelog should be generated correctly

Steps to Test This Pull Request

1. Run cz ch in commitizen (or any project that had merged pull request)

Additional context

#315

[codecov]

Codecov Report

Merging #354 (2756e91) into master (4671258) will increase coverage by 0.24%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #354      +/-   ##
==========================================
+ Coverage   97.29%   97.53%   +0.24%     
==========================================
  Files          35       39       +4     
  Lines         999     1340     +341     
==========================================
+ Hits          972     1307     +335     
- Misses         27       33       +6     

Flag	Coverage Δ
unittests	97.53% <100.00%> (+0.24%)	⬆️

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)
commitizen/changelog.py	95.77% <100.00%> (ø)
commitizen/git.py	96.62% <100.00%> (ø)
commitizen/bump.py	100.00% <0.00%> (ø)
commitizen/exceptions.py	100.00% <0.00%> (ø)
commitizen/__init__.py	100.00% <0.00%> (ø)

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