RepositoryUpdateMirrorWorker::UpdateError: NoMethodError: undefined method `commit_id' for nil:NilClass

https://sentry.gitlap.com/gitlab/gitlabcom/issues/123763/

NoMethodError: undefined method `commit_id' for nil:NilClass
  lib/gitlab/git/operation_service.rb:12:in `from_gitaly'
    branch_update.commit_id,
  lib/gitlab/gitaly_client/operation_service.rb:135:in `user_ff_branch'
    Gitlab::Git::OperationService::BranchUpdate.from_gitaly(branch_update)
  lib/gitlab/git/repository.rb:2236:in `gitaly_ff_merge'
    gitaly_operations_client.user_ff_branch(user, source_sha, target_branch)
  lib/gitlab/git/repository.rb:801:in `block in ff_merge'
    gitaly_ff_merge(user, source_sha, target_branch)
  lib/gitlab/gitaly_client.rb:232:in `block (2 levels) in migrate'
    yield is_enabled
...
(60 additional frame(s) were not displayed)

RepositoryUpdateMirrorWorker::UpdateError: NoMethodError: undefined method `commit_id' for nil:NilClass
  lib/gitlab/sidekiq_status/server_middleware.rb:5:in `call'
    ret = yield
  lib/gitlab/sidekiq_middleware/request_store_middleware.rb:6:in `call'
    yield
  lib/gitlab/sidekiq_middleware/memory_killer.rb:16:in `call'
    yield
  lib/gitlab/sidekiq_middleware/arguments_logger.rb:6:in `call'
    yield
  lib/gitlab/metrics/sidekiq_middleware.rb:13:in `block in call'
    trans.run { yield }
...
(37 additional frame(s) were not displayed)

NoMethodError: undefined method `commit_id' for nil:NilClass