[FEATURE] Skip initial commit message when it's not a real initial commit

[guibranco]

In the dotnet-aicommitmessage project, the following message is currently used for initial commits:

initial commit - [generated description from AI]

However, this pattern should only be applied to true initial commits—not to the first commit on a feature branch that already has history. We need a way to programmatically detect whether a commit is genuinely the initial commit (in the repository or on a fresh orphan branch) and skip applying this default message otherwise.

Acceptance Criteria:

• Detect whether the current commit is the true initial commit (e.g., by checking if there is no parent commit)
• Detect and skip initial commit message if it's the first commit on a feature branch but not the actual root commit
• Ensure the tool continues to generate commit messages for regular commits as expected
• Add unit and integration tests to verify behavior for different commit scenarios