Handle encoding errors for MergeToRefService

What does this MR do?

It handles encoding errors generated when building the Gitaly::UserMergeToRefRequest.

See https://sentry.gitlab.net/gitlab/gitlabcom/issues/793340/

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content