Refactor WebIdeCommitsCount class

What does this MR do?

In preparation for https://gitlab.com/gitlab-org/gitlab-ce/issues/63886 and https://gitlab.com/gitlab-org/gitlab-ce/issues/63887, we're refactoring the logic in WebIdeCommitsCount class to reuse it in other places.

Does this MR meet the acceptance criteria?

Conformity

- [ ] Changelog entry

- [ ] Documentation created/updated or follow-up review issue created

• Code review guidelines
• Merge request performance guidelines
• Style guides

- [ ] Database guides

- [ ] Separation of EE specific content