Feat/git config native

[bernoussama]

Fixes #376

• Replaced git config commands invocations with gix-config native Rust implementation
• Added GixError type
• Added gix_config tests, ensuring same output as git config commands

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[svarlamov]

Thanks for the PR had a few thoughts:

1. git config works even outside of a repo. I noticed that you're passing a filepath into the repo -- what if the user is outside of a repo? Will this still work?
2. Please use the existing TestRepo harness instead of creating another utility for test repos for the unit tests. You can reference the existing unit tests in the codebase to see how to use TestRepo
3. Noticed the tests use the serial macro. We try to avoid using it if at all possible -- with TestRepo maybe we could avoid it?

[bernoussama]

Hey @svarlamov ,

1. Yes it should work if not inside a repo it'll fallback to env and global config as per gix_config docs, but it should be tested, will include a test for it, good observation.
2. I will look into TestRepo and use it for testing
3. For the serial macro, I'll use TestRepo for tests and see how it goes

[bernoussama]

about current git config implementation:

• It'll work outside of a repo, as i added a fallback to global Git config file if not inside a Git repo. This scenario remains untested however, because it would involve modifying env variables requiring these tests to be serial.

[svarlamov]

LGTM! I added some tests for bare clones, etc. but it was pretty complete. Thanks!