Use git CLI for version metadata

[8Keep]

The author of the gradle plugin we use (that uses jgit) has abandoned it, saying it's easy to just use git cli from gradle directly: https://andrewoberstar.com/posts/2024-04-02-dont-commit-to-grgit/

I ran into a broken version info when trying to speed up the gradle builds, because I use git worktrees to get work done, and jgit doesn't seem to support git worktrees. So this enables new AI native functionality: many git worktrees for different workstreams in parallel, too.

Split out from #2701

[gemini-code-assist]

Code Review

This pull request replaces the grgit library with direct Git command execution using Gradle's providers.exec to retrieve versioning metadata. The changes include removing the gradle-git dependency and refactoring version.gradle to use a new gitOutput helper. A review comment identifies a potential issue where jmeBranchName could be overwritten with an empty string if Git commands fail, suggesting a fallback to the default 'unknown' value.

[8Keep]

Ready for review

[github-actions]

🖼️ Screenshot tests have failed.

The purpose of these tests is to ensure that changes introduced in this PR don't break visual features. They are visual unit tests.

📄 Where to find the report:

• Direct Download: screenshot-test-report
• Alternatively, go to the failed run summary > Artifacts > screenshot-test-report
• Download the zip and open jme3-screenshot-tests/build/reports/ScreenshotDiffReport.html

⚠️ If you didn't expect to change anything visual:
Fix your changes so the screenshot tests pass.

✅ If you did mean to change things:
Review the replacement images in jme3-screenshot-tests/build/changed-images to make sure they really are improvements and then replace and commit the replacement images at jme3-screenshot-tests/src/test/resources.

✨ If you are creating entirely new tests:
Find the new images in jme3-screenshot-tests/build/changed-images and commit the new images at jme3-screenshot-tests/src/test/resources.

Note; it is very important that the committed reference images are created on the build pipeline, locally created images are not reliable. Similarly tests will fail locally but you can look at the report to check they are "visually similar".

See https://github.com/jMonkeyEngine/jmonkeyengine/blob/master/jme3-screenshot-tests/README.md for more information

Contact @richardTingle (aka richtea) for guidance if required

[8Keep]

@richardTingle it seems like the screenshot tests are very flaky recently.

[riccardobl]

@gemini-code-assist review

[gemini-code-assist]

Code Review

This pull request replaces the grgit library with direct Git command execution using Gradle's providers.exec to retrieve versioning information, effectively removing the gradle-git dependency. The logic for determining the revision, branch name, and tags has been refactored to use shell commands. Review feedback suggests improving the robustness of the version tag extraction and adjusting the regex for matching Git hashes to account for potential length variations in git describe output.

[8Keep]

@riccardobl should be good now