ref-filter: restore prefix-scoped iteration

dabecb9 (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.

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 these single-kind filters, construct the iterator with the namespace
prefix when start_after is not set. Keep the existing unscoped
construction for start_after, whose seek position may differ from the
namespace prefix.

With 10,000 unrelated loose refs, 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

Link: https://lore.kernel.org/git/aGZidwwlToWThkn8@pks.im/
Link: https://lore.kernel.org/git/xmqqikjq7s16.fsf@gitster.g/
Link: https://lore.kernel.org/r/CAOLa=ZRHKNNymXGk31YgECjUmF9nZ8GsPUdQb7aKBH5DKMz7=w@mail.gmail.com
Fixes: dabecb9 ("for-each-ref: introduce a '--start-after' option")
Suggested-by: Karthik Nayak <karthik.188@gmail.com>
Assisted-by: Codex gpt-5.5
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: tamird

ref-filter.c

@@ -3316,15 +3316,14 @@ static int do_filter_refs(struct ref_filter *filter, unsigned int type, refs_for
 
 	if (prefix) {
 		struct ref_iterator *iter;
+		struct ref_store *store = get_main_ref_store(the_repository);
 
-		iter = refs_ref_iterator_begin(get_main_ref_store(the_repository),
-					       "", NULL, 0, 0);
-
-		if (filter->start_after)
+		if (filter->start_after) {
+			iter = refs_ref_iterator_begin(store, "", NULL, 0, 0);
 			ret = start_ref_iterator_after(iter, filter->start_after);
-		else
-			ret = ref_iterator_seek(iter, prefix,
-						REF_ITERATOR_SEEK_SET_PREFIX);
+		} else {
+			iter = refs_ref_iterator_begin(store, prefix, NULL, 0, 0);
+		}
 
 		if (!ret)
 			ret = do_for_each_ref_iterator(iter, fn, cb_data);

t/perf/p6300-for-each-ref.sh

@@ -1,6 +1,6 @@
 #!/bin/sh
 
-test_description='performance of for-each-ref'
+test_description='performance of ref-filter users'
 . ./perf-lib.sh
 
 test_perf_fresh_repo
@@ -84,4 +84,41 @@ test_expect_success 'pack refs' '
 '
 run_tests "packed"
 
+test_expect_success REFFILES 'setup many unrelated loose refs' '
+	git init scoped &&
+	test_commit -C scoped --no-tag base &&
+	test_seq $ref_count_per_type |
+		sed "s,.*,update refs/custom/unrelated_& HEAD," |
+		git -C scoped update-ref --stdin &&
+	git -C scoped update-ref refs/remotes/origin/main HEAD &&
+	git -C scoped update-ref refs/tags/only HEAD
+'
+
+test_perf "branch (many unrelated loose refs)" --prereq REFFILES "
+	(
+		cd scoped &&
+		for i in \$(test_seq $test_iteration_count); do
+			git branch --format='%(refname)' >/dev/null
+		done
+	)
+"
+
+test_perf "branch --remotes (many unrelated loose refs)" --prereq REFFILES "
+	(
+		cd scoped &&
+		for i in \$(test_seq $test_iteration_count); do
+			git branch --remotes --format='%(refname)' >/dev/null
+		done
+	)
+"
+
+test_perf "tag (many unrelated loose refs)" --prereq REFFILES "
+	(
+		cd scoped &&
+		for i in \$(test_seq $test_iteration_count); do
+			git tag --format='%(refname)' >/dev/null
+		done
+	)
+"
+
 test_done