fix(config)!: nested environment config overrides

[mackness]

Description

Behavior related to overriding configuration for nested elements via environment variables changed in config-rs@v0.9. We need to use __ instead of _ separator to match nested fields. For details see mehcode/config-rs#73.

Also added some documentation that mentions it's possible to use environment variables to override configuration.

How Has This Been Tested?

All tests are passing locally, I updated config::test::parse_config and added a couple more env override assertions. I also tested in a small sandbox project and observed the following:

history

commit 9d5ce9759e2fbedc48fbb23e22cdad74214c809a (HEAD -> main, tag: v1.0.3-rc1)
Author: Mack Solomon <macksol@gmail.com>
Date:   Tue Apr 18 22:14:30 2023 -0700

    feat(all): a feature

commit f5b5135793de7bacfdec9750ef9d535320aaa614 (tag: v1.0.2)
Author: Mack Solomon <macksol@gmail.com>
Date:   Tue Apr 18 22:13:35 2023 -0700

    fix(all): a fix

result (with fix)

➜  release-playground git:(main) ✗ export CLIFF_GIT__GIT__IGNORE_TAGS="v[0-9]+.[0-9]+.[0-9]+-rc[0-9]+"    
➜  release-playground git:(main) ✗ ~/dev/git-cliff/target/release/git-cliff --latest                  
# Changelog

All notable changes to this project will be documented in this file.

## [1.0.2] - 2023-04-19

### Bug Fixes

- A fix

<!-- generated by git-cliff -->

We can see that the first RC tag in history is being ignored as expected so the env override worked! 🎊

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (no code change)
• Refactor (refactoring production code)
• Other

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.
• I have formatted the code with rustfmt.
• I checked the lints with clippy.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.44 ⚠️

Comparison is base (2d72125) 44.08% compared to head (070dbc1) 43.64%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #157      +/-   ##
==========================================
- Coverage   44.08%   43.64%   -0.44%     
==========================================
  Files          12       12              
  Lines         565      573       +8     
==========================================
+ Hits          249      250       +1     
- Misses        316      323       +7     

Flag	Coverage Δ
unit-tests	43.64% <100.00%> (-0.44%)	⬇️

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

Impacted Files	Coverage Δ
git-cliff-core/src/config.rs	37.50% <100.00%> (-2.50%)	⬇️

... and 2 files with indirect coverage changes

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[orhun]

LGTM!