ref-filter: restore prefix-scoped iteration

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>