fix: compare BASE with merged state instead of HEAD directly

[swade1987]

This PR merges all improvements and fixes from the next branch into main, completing the work to enhance the kustomize-diff GitHub Action. The next branch has been used as a staging area to validate multiple iterations of fixes needed to make the kustomize-diff action work reliably across all GitHub Actions environments.

Key improvements included in this merge

1. Merged state comparison

• The action now correctly compares the BASE branch with the result of merging HEAD into BAS.E
• This accurately represents what would change when the PR is merged.
• Fixes incorrect behavior where parallel changes to BASE would appear to be reverted

2. Branch reference handling

• Added robust branch reference resolution that works with local branches, remote branches, and commit SHAs
• Fixed issues with detached HEAD states in GitHub Actions environments
• Correctly handles references regardless of format (branch name vs. remote/branch)

3. Git identity configuration

• Added git user identity configuration to enable merge operations in CI environments
• Prevents "Committer identity unknown" errors during the temporary merge process

4. Enhanced error handling and debugging

• Improved error messages for easier troubleshooting
• Added detailed debug logging
• Better cleanup of temporary resources

Testing
This implementation has been thoroughly tested across multiple repositories and PR scenarios, including:

1. PRs where BASE has parallel changes
2. PRs from branches created without the latest BASE changes
3. Different repository structures and kustomization setups

Related Issues
Closes #11

[swade1987]

🎉 This PR is included in version 0.3.0 🎉

The release is available on GitHub release

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