ref-filter: avoid backend filtering with --ignore-case

ref-filter: avoid backend filtering with --ignore-case

When for-each-ref is used with --ignore-case, we expect
match_name_as_path() to do a case-insensitive match. But
there's an extra layer of filtering that happens before we
even get there. Since commit cfe004a5a9 (ref-filter: limit
traversal to prefix, 2017-05-22), we feed the prefix to the
ref backend so that it can optimize the ref iteration.

There's no mechanism for us to tell the backend we're matching
case-insensitively.  Nor is there likely to be one anytime soon,
since the packed backend relies on binary-searching the sorted list
of refs. Let's just punt on this case. The extra filtering is an
optimization that we simply can't do. We'll still give the correct
answer via the filtering in match_name_as_path().

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>