`Rugged::Reference.valid_name?` used in
`Gitlab::GitRefValidator.validate` fails on strings containing null
bytes because it uses `StringValueCStr()`. Per
https://silverhammermba.github.io/emberb/c/:

Ruby’s String kinda corresponds to C’s char*. The simplest macro is
StringValueCStr() which returns a null-terminated char* for a
String. The problem here is that a Ruby String might contain nulls - in
which case StringValueCStr() will raise an ArgumentError!

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/54466