Branch with a commit detection defaults to Gitaly

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)

changelogs/unreleased/zj-branch-containing-sha-opt-out.yml

+---
+title: Detecting branchnames containing a commit uses Gitaly by default
+merge_request:
+author:
+type: performance

lib/gitlab/git/repository.rb

@@ -1385,7 +1385,8 @@ module Gitlab
       end
       def branch_names_contains_sha(sha)
-        gitaly_migrate(:branch_names_contains_sha) do |is_enabled|
+        gitaly_migrate(:branch_names_contains_sha,
+                      status: Gitlab::GitalyClient::MigrationStatus::OPT_OUT) do |is_enabled|
           if is_enabled
             gitaly_ref_client.branch_names_contains_sha(sha)
           else