fix: Raise NotAGitProjectError only in git related command

[Lee-W]

Only Running cz commit, cz bump and cz changelog in a non git project will raise NotAGitProjectError
Other commands like cz --help will work now

#206

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Types of changes

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• Bugfix

Description

Describe what the change is

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

1. Run cz --help, cz ls, cz example, cz info, cz init, cz version, cz check
2. Run cz commit, cz bump, cz changelog

Expected behavior

1. After running the first group (i.e. cz --help ) of commands, no error should be raised.
2. After running the second group (i.e. cz commit ) of commands, no error should be raised.

Related Issue

#206

Additional context

Add any other context or screenshots about the pull request here.

[codecov]

Codecov Report

Merging #219 into master will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #219      +/-   ##
==========================================
+ Coverage   95.64%   95.69%   +0.05%     
==========================================
  Files          34       34              
  Lines         942      953      +11     
==========================================
+ Hits          901      912      +11     
  Misses         41       41              

Flag	Coverage Δ
#unittests	95.69% <100.00%> (+0.05%)	⬆️

Impacted Files	Coverage Δ
commitizen/commands/bump.py	91.66% <100.00%> (+0.20%)	⬆️
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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[woile]

Fantastic!