Optimize network graph by skipping expensive commit centering

What does this MR do and why?

Contributes to https://gitlab.com/gitlab-com/gl-infra/production-engineering/-/issues/27625

Problem

The network graph's count_to_display_commit_in_center method performs an expensive operation to center a specific commit in the visualization by repeatedly fetching batches of 650 commits until it finds the target commit's position.

Solution

When the optimize_network_graph_calculations feature flag is enabled, use Repository#count_commits (git rev-list --count) to efficiently calculate the distance between a commit and the head ref, instead of iterating through commits one by one.

The unused find_commits method is also removed as it has been superseded by list_commits.

References

https://gitlab.com/gitlab-com/gl-infra/production-engineering/-/issues/27625

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.