backfill: auto-detect sparse-checkout from config

Commit 85127bc ("backfill: assume --sparse when sparse-checkout is
enabled") intended for 'git backfill' to consult the repository
configuration when the user does not pass '--sparse' or
'--no-sparse' on the command line. It added the sentinel check:

    if (ctx->sparse < 0)
        ctx->sparse = cfg->apply_sparse_checkout;

However, the ctx->sparse field is initialized to 0 instead of -1,
so this guard never triggers. Consequently, the repository config
(core.sparseCheckout) is never checked, and the command always
performs a full backfill even when sparse-checkout is enabled.

Fix this by initializing ctx->sparse to -1, ensuring the existing
fallback logic correctly reads the repository configuration when
no explicit flags are provided.

Add a test to verify that 'git backfill' automatically respects
sparse-checkout settings when no flags are passed.

Signed-off-by: Trieu Huynh <vikingtc4@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: trieu1162000

builtin/backfill.c

@@ -120,7 +120,7 @@ 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,
 	};
 	struct option options[] = {
 		OPT_INTEGER(0, "min-batch-size", &ctx.min_batch_size,

t/t5620-backfill.sh

@@ -119,6 +119,21 @@ 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 		\