Omit max-count for diverging_commit_counts behind feature flag

What does this MR do?

part of: gitaly#1462 (closed)

Now that commit graphs are enabled in gc gitaly!1218 (merged), the --max-count argument shouldn't be necessary in most cases. However, before getting rid of it completely, we want to test this out behind a feature flag, as for very large repos we may still need --max-count (gitaly#1462 (comment 167371813))

This MR sets max_count to 0, which in Gitaly will result in omitting the --max-count flag:

https://gitlab.com/gitlab-org/gitaly/blob/master/internal/service/commit/count_diverging_commits.go#L53-54

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
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• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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