Add parentBranch configuration to support CI environments with detached HEAD

[tinder-ravitejabonagiri]

Add Support for Explicit Parent Branch in ForkCommit

Problem

When running in CI environments where repositories are cloned with detached HEAD (common in PR builds), the ForkCommit provider's automatic parent branch detection can fail. This is because git show-branch -a might not show correct branch information in detached HEAD state, leading to "Parent branch not found" errors.

Solution

Added the ability to explicitly specify a parent branch in the ForkCommit provider through configuration. This makes the tool more reliable in CI environments by removing the dependency on automatic branch detection.

Changes

1. Added parentBranch field to CommitShaProviderConfiguration
2. Added parentBranch configuration option to AffectedModuleConfiguration
3. Modified ForkCommit to use provided parent branch if available, falling back to auto-detection if not provided

Configuration Example

affectedModuleDetector {
    compareFrom = "ForkCommit"
    parentBranch = "main"  // Explicitly specify the parent branch
}

Usage in CI

Instead of relying on automatic detection which might fail:

# This might fail in detached HEAD state
./gradlew test -Paffected_module_detector.enable

You can now explicitly specify the parent branch:

# This works reliably even in detached HEAD state
./gradlew test -Paffected_module_detector.enable -Paffected_module_detector.parentBranch=main

Benefits

1. Reliability: Works consistently in CI environments with detached HEAD
2. Flexibility: Users can explicitly specify which branch to compare against
3. Backward Compatible: Falls back to existing auto-detection if no parent branch is provided
4. CI/CD Friendly: Makes it easier to configure in CI/CD pipelines where branch information might be limited

Documentation

Updated README.md to include:

• New parentBranch configuration option
• Example usage in CI environments
• Explanation of when to use explicit parent branch configuration

Breaking Changes

None. This is a backward-compatible enhancement that maintains existing behavior when parentBranch is not specified.

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[tinder-ravitejabonagiri]

@joshafeinberg, I would appreciate your review on these changes.