Verify commit presence before Changelog generation

What does this MR do and why?

• Contributes to #340941
• Follow-up for: !89032 (merged)

Problem

Method '#diverging_commit_count' raises an exception when provided commits are missing in the repository.

Solution

Verify that commits are present in the repository.

Screenshots or screen recordings

How to set up and validate locally

1. Visit http://127.0.0.1:3000/api/v4/projects/1/repository/changelog?version=v14.1.2&from=0000
2. You should see an human-friendly error instead of "Internal server error".

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.