Rewrite Rugged::Reference.valid_name? in Ruby

What does this MR do and why?

Previously this code was written in libgit2, which forced GitLab to depend on Rugged. Rewrite this code in Ruby.

References:

git-core:

• https://github.com/git/git/blob/f06d47e7e0d9db709ee204ed13a8a7486149f494/refs.c#L36-L68

libgit2:

• https://github.com/libgit2/libgit2/blob/v1.9.1/src/libgit2/refs.c#L1418-L1425
• https://github.com/libgit2/libgit2/blob/v1.9.1/src/libgit2/refs.c#L908-L1040

Relates to #430709

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