Encoding::UndefinedConversionError: U+53CB from UTF-8 to ASCII-8BIT

• https://sentry.gitlab.net/gitlab/gitlabcom/issues/793340/
• https://sentry.gitlab.net/gitlab/gitlabcom/issues/793990

Encoding::UndefinedConversionError: U+53CB from UTF-8 to ASCII-8BIT
  gitlab/gitaly_client/operation_service.rb:104:in `initialize'
    request = Gitaly::UserMergeToRefRequest.new(
  gitlab/gitaly_client/operation_service.rb:104:in `new'
    request = Gitaly::UserMergeToRefRequest.new(
  gitlab/gitaly_client/operation_service.rb:104:in `user_merge_to_ref'
    request = Gitaly::UserMergeToRefRequest.new(
  gitlab/git/repository.rb:541:in `block in merge_to_ref'
    gitaly_operation_client.user_merge_to_ref(user, source_sha, branch, target_ref, message)
  gitlab/git/wraps_gitaly_errors.rb:7:in `wrapped_gitaly_errors'
    yield block
...
(168 additional frame(s) were not displayed)

Encoding::UndefinedConversionError: U+53CB from UTF-8 to ASCII-8BIT