Compute repository checksum by default

Acceptance testing was done in:
https://gitlab.com/gitlab-org/gitaly/issues/1146. Errors were limited to
NotFound errors, as the repository really doesn't exists.

Closes https://gitlab.com/gitlab-org/gitaly/issues/1146

Optimistically moved https://gitlab.com/gitlab-org/gitaly/issues/1105 to
opt out on the Gitaly migration board.

changelogs/unreleased/zj-repo-checksum-opt-out.yml

+---
+title: Compute Gitlab::Git::Repository#checksum on Gitaly by default
+merge_request:
+author:
+type: performance

lib/gitlab/git/repository.rb

@@ -1546,7 +1546,8 @@ module Gitlab
       end
 
       def checksum
-        gitaly_migrate(:calculate_checksum) do |is_enabled|
+        gitaly_migrate(:calculate_checksum,
+                      status: Gitlab::GitalyClient::MigrationStatus::OPT_OUT) do |is_enabled|
           if is_enabled
             gitaly_repository_client.calculate_checksum
           else