MergeRequestsController conflicts may fail with ISO-8859 data

https://sentry.gitlap.com/gitlab/gitlabcom/issues/10410/

Encoding::UndefinedConversionError: "\xC3" from ASCII-8BIT to UTF-8
  app/controllers/projects/merge_requests_controller.rb:142:in `block (2 levels) in conflicts'
    render json: @merge_request.conflicts
  app/controllers/projects/merge_requests_controller.rb:137:in `conflicts'
    respond_to do |format|
  lib/gitlab/request_profiler/middleware.rb:15:in `call'
    @app.call(env)
  lib/gitlab/middleware/rails_queue_duration.rb:20:in `call'
    @app.call(env)
  lib/gitlab/metrics/rack_middleware.rb:18:in `block in call'
    retval = trans.run { @app.call(env) }
...
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