test: improve git isolation and fixes test missing a repository

[noirbizarre]

Description

This PR follows #638 and extend the isolation to any GIT_ prefixed variables. It also ensure that every test requiring a tmp_git_project or tmp_commitizen_project fixture has it, even for test expecting a failure (to ensure the proper failure).

This fixes some false negatives in pre-commit as well as ordering side-effects for tests that were missing the fixture like this build failing on some totally unrelated (and untouched) cases.

There is few misc fixes and redundancy removal (lots of unnecessary nested .as_cwd() calls as well as some fixtures extracted into @pytest.mark.usefixtures() when not used in the test but still requiring the fixture setup))

Checklist

• Add test cases to all the changes you introduce (N/A)
• Run ./scripts/format and ./scripts/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 (N/A)

[codecov]

Codecov Report

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Additional details and impacted files

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[Lee-W]

Just one minor question. I think this one is almost ready to merge

[noirbizarre]

Yes, #638 was using a temporary GIT_CONFIG_GLOBAL ensuring the clean configuration.
This PR extends the isolation to all GIT_ environment variables because some settings can be defined by both the configuration file and the environment.
pre-commit is interfering with some of those, the git command is also exporting some depending on the version you use (see: https://github.com/pre-commit/pre-commit/blob/1f59f4cba8241ed7e572a1b5818bba74c6f31181/pre_commit/git.py#L27-L48), your shell can export some, your custom promt too, github actions can set some...
That's why without isolation you may have different behavior and some false negative depending on wether you are running in pre-commit or not, depending on your pre-commit version, depending on your git version, or depending of your own custom shell...
With this change, you ensure consistent and reproducible behavior on all environments, they always run with the minimal configuration (author and email) and no setting defined outside of the test case

[Lee-W]

@noirbizarre Cool! many thanks for the detailed explanation!