Branch with a commit detection defaults to Gitaly
What does this MR do?
By default this was an OPT_IN RPC, and now graduates to OPT_OUT as the acceptance testing was successful in: https://gitlab.com/gitlab-org/gitaly/issues/1008 This follows the conventions of our [migration process](https://gitlab.com/gitlab-org/gitaly/blob/master/doc/MIGRATION_PROCESS.md)
Does this MR meet the acceptance criteria?
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Changelog entry added, if necessary - Review
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Has been reviewed by Backend
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What are the relevant issue numbers?
Part of gitaly#884 (closed)