Error 500 resolving conflicts: Encoding::UndefinedConversionError: U+00F1 from UTF-8 to ASCII-8BIT

https://sentry.gitlab.net/gitlab/gitlabcom/issues/779121/

Encoding::UndefinedConversionError: U+00F1 from UTF-8 to ASCII-8BIT
  gitlab/gitaly_client/conflicts_service.rb:63:in `initialize'
    Gitaly::ResolveConflictsRequestHeader.new(
  gitlab/gitaly_client/conflicts_service.rb:63:in `new'
    Gitaly::ResolveConflictsRequestHeader.new(
  gitlab/gitaly_client/conflicts_service.rb:63:in `resolve_conflicts_request_header'
    Gitaly::ResolveConflictsRequestHeader.new(
  gitlab/gitaly_client/conflicts_service.rb:43:in `block in resolve_conflicts'
    header = resolve_conflicts_request_header(target_repository, resolution, source_branch, target_branch)
  grpc/generic/active_call.rb:397:in `each'
    requests.each { |r| @call.run_batch(SEND_MESSAGE => @marshal.call(r)) }
...
(133 additional frame(s) were not displayed)

Encoding::UndefinedConversionError: U+00F1 from UTF-8 to ASCII-8BIT