fix: compare BASE with merged state instead of HEAD directly

[swade1987]

This PR addresses issue #11, where the action incorrectly indicated that changes to the BASE branch would be reverted when the PR is merged.

Problem

The previous implementation compared the kustomize output from the BASE branch directly with the HEAD branch. This approach fails to account for git's merge behaviour and can lead to misleading diffs:

Old Base -> BASE
         \
           -> HEAD

When someone creates a feature branch without first pulling the latest changes, parallel changes made to BASE would appear as if they would be reverted by the PR, even though git's actual merge behavior would preserve them.

Solution

Instead of comparing BASE with HEAD directly, this PR:

1. Builds the kustomize output for the BASE branch
2. Creates a temporary merge branch from BASE
3. Merges HEAD into this temporary branch (simulating what git would do during PR merge)
4. It builds the kustomize output for the merged state
5. Compares the BASE output with the merged output

This approach shows only the changes introduced by merging the PR, providing a more accurate representation of the final state.

Additional Improvements

1. Added proper merge conflict detection and handling
2. Improved cleanup of temporary git branches
3. Enhanced debug logging for the merge process
4. Maintained backward compatibility with existing input parameters

Closes #11

How has this been tested?

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)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s

[joshgubler]

This looks great! Thank you!

[swade1987]

🎉 This PR is included in version 0.3.0-next.1 🎉

The release is available on GitHub release

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