This regex intended to check if any refs that do not contain a slash
were a prefix of a ref containing the slash, plus the slash. That is:
`abc` is a prefix of `abc/def`, but not of `abcd/ef`.

However, the regex grouping was incorrect, and we were creating a regex
like `%r{^abc|def/}`, which doesn't contain parentheses, and so would
actually match plain `abc` (it wouldn't match `def/`).