From: Nikolaus Schuetz <nikolauspschuetz@gmail.com>

git-check-ref-format(1) documents that a refname cannot contain a
space, tilde, caret, colon, question-mark, asterisk or open-bracket,
and that it cannot be the single character "@".  Of these, only "?"
was tested as a character embedded in an otherwise-valid refname;
"*" was checked only as a lone character or with --refspec-pattern.

Add the remaining forbidden characters in that embedded form, and
check that "@" alone is rejected even with --allow-onelevel -- where
"@" is otherwise a valid refname component, as "refs/@" confirms.

Signed-off-by: Nikolaus Schuetz <nikolauspschuetz@gmail.com>

Submitted-As: https://lore.kernel.org/git/pull.2203.git.1786653837190.gitgitgadget@gmail.com