Merge branch 'th/backfill-auto-detect-sparseness-fix'

"git backfill" is capable of auto-detecting a sparsely checked out
working tree, which was broken.

* th/backfill-auto-detect-sparseness-fix:
  backfill: auto-detect sparse-checkout from config

builtin/backfill.c

@@ -119,7 +119,7 @@ int cmd_backfill(int argc, const char **argv, const char *prefix, struct reposit
 		.repo = repo,
 		.current_batch = OID_ARRAY_INIT,
 		.min_batch_size = 50000,
-		.sparse = 0,
+		.sparse = -1,
 		.revs = REV_INFO_INIT,
 	};
 	struct option options[] = {

t/t5620-backfill.sh

@@ -175,6 +175,21 @@ test_expect_success 'backfill --sparse' '
 	test_line_count = 0 missing
 '
 
+test_expect_success 'backfill auto-detects sparse-checkout from config' '
+	git clone --sparse --filter=blob:none \
+		--single-branch --branch=main \
+		"file://$(pwd)/srv.bare" backfill-auto-sparse &&
+
+	git -C backfill-auto-sparse rev-list --quiet --objects --missing=print HEAD >missing &&
+	test_line_count = 44 missing &&
+
+	GIT_TRACE2_EVENT="$(pwd)/auto-sparse-trace" git \
+		-C backfill-auto-sparse backfill &&
+
+	test_trace2_data promisor fetch_count 4 <auto-sparse-trace &&
+	test_trace2_data path-walk paths 5 <auto-sparse-trace
+'
+
 test_expect_success 'backfill --sparse without cone mode (positive)' '
 	git clone --no-checkout --filter=blob:none		\
 		--single-branch --branch=main 		\