Fix Encoding::UndefinedConversionError in RemoteService

What does this MR do and why?

Contributes to #567713 (closed)

Problem

RemoteService#update_remote_mirror fails with Unicode branch names.

The error occurred when passing UTF-8 branch names directly to Gitaly protobuf initialization without proper encoding conversion.

Solution

Encode branch names using encode_binary before Gitaly requests.

References

Encoding::UndefinedConversionError in RemoteSer... (#567713 - closed)

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Related to #567713 (closed)