Encoding bug in ListConflictFiles

https://log.gitlap.com/app/kibana#/doc/gitaly-*/gitaly-2018.03.22/gitlab?id=AWJN_U3eNBepG3IEeKsL&_g=()

rpc error: code = Unknown desc = Encoding::UndefinedConversionError: U+00F3 from UTF-8 to ASCII-8BIT

Sentry event: https://sentry.gitlap.com/gitlab/gitaly-production/issues/140372/events/5412819/

/cc @ahmadsherif