Prevents refspec as branch name, which would bypass branch protection
when used in conjunction with rebase.
HEAD seems to be a special case with lots of occurrence,
so it is considered valid for now.
Another special case is `refs/head/*`, which can be imported.
`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
Mark Chao