ref-filter: restore prefix-scoped iteration

dabecb9db2 (for-each-ref: introduce a '--start-after' option, 2025-07-15) changed branch, remote-tracking branch, and tag enumeration from constructing an iterator with the namespace prefix to constructing an unscoped iterator and seeking to the prefix. Review of --start-after noted that the construction prefix and seek position represent different state and are easy to conflate [1]. It also noted that future branch or tag support would need to retain the namespace prefix while moving the cursor [2]. The files backend constructs its loose-ref iterator with cache priming enabled. cache_ref_iterator_begin() immediately applies the construction prefix through cache_ref_iterator_set_prefix(), reading loose refs beneath it before packed refs are opened. An empty prefix therefore reads every loose ref, and a later seek cannot undo that I/O. For the current single-kind filters, construct the iterator with the namespace prefix when start_after is not set. Leave the existing start_after path unchanged; no current command combines it with these filters, and future support must carry the prefix separately from the cursor. With 10,000 unrelated loose refs in the files backend, the p6300 tests improve as follows: before after branch 2.74 s 0.11 s branch --remotes 2.81 s 0.12 s tag 3.01 s 0.11 s [1] https://lore.kernel.org/r/aGZidwwlToWThkn8@pks.im/ [2] https://lore.kernel.org/r/xmqqikjq7s16.fsf@gitster.g/ Fixes: dabecb9db2b2 ("for-each-ref: introduce a '--start-after' option") Suggested-by: Karthik Nayak <karthik.188@gmail.com> Signed-off-by: Tamir Duberstein <tamird@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>