Projects::MergeRequests::ConflictsController#show: Encoding::UndefinedConversionError: "\xE5" from ASCII-8BIT to UTF-8

https://sentry.gitlab.net/gitlab/gitlabcom/issues/811460/

Test case: https://gitlab.com/ddbxyrj/test/merge_requests/1/conflicts

Encoding::UndefinedConversionError: "\xE5" from ASCII-8BIT to UTF-8
  active_support/core_ext/object/json.rb:36:in `encode'
    super(options)
  active_support/core_ext/object/json.rb:36:in `to_json'
    super(options)
  active_support/core_ext/object/json.rb:36:in `to_json'
    super(options)
  active_support/json/encoding.rb:55:in `to_json'
    super.gsub ESCAPE_REGEX_WITH_HTML_ENTITIES, ESCAPED_CHARS
  json/common.rb:223:in `generate'
    state.generate(obj)
...
(147 additional frame(s) were not displayed)

Encoding::UndefinedConversionError: "\xE5" from ASCII-8BIT to UTF-8