Fix git message parsing

[Lee-W]

Description

originally, we use "----------commit-delimiter----------" to split git command result and strip them which should be "\n----------commit-delimiter----------\n" instead.

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

Proceed as usual if users do not provide their email or commit message in their commit history

Steps to Test This Pull Request

1. Create an empty project
2. git init
3. touch test-file
4. git commit --author="Name <>" -m "whatever"
5. touch another-test-file
6. git commit --author="Name <>" -m "" --allow-empty

Additional context

#340

[codecov]

Codecov Report

Merging #341 (f01a03c) into master (1a33099) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #341      +/-   ##
==========================================
+ Coverage   97.20%   97.29%   +0.09%     
==========================================
  Files          35       35              
  Lines        1000      999       -1     
==========================================
  Hits          972      972              
+ Misses         28       27       -1     

Flag	Coverage Δ
unittests	97.29% <100.00%> (+0.09%)	⬆️

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

Impacted Files	Coverage Δ
commitizen/cli.py	97.50% <ø> (ø)
commitizen/commands/init.py	91.66% <ø> (ø)
commitizen/__version__.py	100.00% <100.00%> (ø)
commitizen/commands/bump.py	94.44% <100.00%> (ø)
commitizen/config/yaml_config.py	100.00% <100.00%> (ø)
commitizen/git.py	96.62% <100.00%> (+1.07%)	⬆️

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