refs: improve verification for-each-ref options

refs: improve verification for-each-ref options

Improve verification of the passed-in for-each-ref options:

  - Require that the `refs` store must be given. It's arguably very
    surprising that we simply return successfully in case the ref store
    is a `NULL` pointer.

  - When expected to trim ref prefixes we will `BUG()` in case the
    refname would become empty or in case we're expected to trim a
    longer prefix than the refname is long. As such, this case is only
    guaranteed to _not_ `BUG()` in case the caller also specified a
    prefix. And furthermore, that prefix must end in a trailing slash,
    as otherwise it may produce an exact match that could lead us to
    trim to the empty string.

An audit shows that there are no callsites that rely on either of these
behaviours, so this should not result in a functional change.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>