vcs Module Fails on CI: Replace logs/HEAD with rev-list HEAD --count for Repo Check

The vcs module in Qbs currently relies on the logs/HEAD file to check if a Git repository is empty. However, during CI builds, the logs/HEAD file does not exist, causing the check to fail. To solve this, I propose replacing the check with git rev-list HEAD --count, which works reliably in both local and CI environments.