Quick fix to identify git repo, worktree or submodule

[Yaswant Pradhan (yaswant)]

PR Summary

umdp3 checker fails to detect the CMS type if the repository in source directory is pointing to a subtree.
In a Git worktree or a git submodule, .git is a regular text file containing a path pointer to the actual .git storage directory. The quick fix safely evaluates to True in both scenarios.

Code Reviewer: Sam Clarke-Green (@t00sa)

Code Quality Checklist

• I have performed a self-review of my own code
• My code follows the project's style guidelines
• Comments have been included that aid understanding and enhance the readability of the code
• My changes generate no new warnings
• All automated checks in the CI pipeline have completed successfully

Testing

• This change has been tested appropriately (locally tested)

Security Considerations

• I have reviewed my changes for potential security issues
• Sensitive data is properly handled (if applicable)
• Authentication and authorisation are properly implemented (if applicable)

AI Assistance and Attribution

• [NA] Some of the content of this change has been produced with the assistance of Generative AI tool name (e.g., Met Office Github Copilot Enterprise, Github Copilot Personal, ChatGPT GPT-4, etc) and I have followed the Simulation Systems AI policy (including attribution labels)

Code Review

• All dependencies have been resolved
• Related Issues have been properly linked and addressed
• Code quality standards have been met
• Tests are adequate and have passed
• Security considerations have been addressed
• Performance impact is acceptable