CompareController: ArgumentError: string contains null byte

https://sentry.gitlab.net/gitlab/gitlabcom/issues/670441/

ArgumentError: string contains null byte
  active_record/connection_adapters/postgresql/quoting.rb:19:in `escape_string'
    @connection.escape(s)
  active_record/connection_adapters/postgresql/quoting.rb:19:in `quote_string'
    @connection.escape(s)
  active_record/connection_adapters/abstract/quoting.rb:175:in `_quote'
    "'#{quote_string(value.to_s)}'"
  active_record/connection_adapters/postgresql/quoting.rb:98:in `_quote'
    super
  active_record/connection_adapters/abstract/quoting.rb:23:in `quote'
    _quote(value)
...
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ArgumentError: string contains null byte