Fix encoding of patterns in Gitaly list_refs calls

What does this MR do and why?

Problem

Reference patterns passed to Gitaly's ListRefsRequest were not properly encoded, causing issues when checking for the existence of branches and tags containing non-ASCII characters.

Exception example:

Encoding::UndefinedConversionError:
 U+00C4 from UTF-8 to ASCII-8BIT

Solution

Encode all patterns as binary using #encode_binary() before sending them to Gitaly.

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