submodule: sparse-index should not collapse links

A submodule is stored as a "Git link" that actually points to a commit
within a submodule. Submodules are populated or not depending on
submodule configuration, not sparse-checkout. To ensure that the
sparse-index feature integrates correctly with submodules, we should not
collapse a directory if there is a Git link within its range.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sparse-index.c

@@ -52,6 +52,7 @@ static int convert_to_sparse_rec(struct index_state *istate,
 		struct cache_entry *ce = istate->cache[i];
 
 		if (ce_stage(ce) ||
+		    S_ISGITLINK(ce->ce_mode) ||
 		    !(ce->ce_flags & CE_SKIP_WORKTREE))
 			can_convert = 0;
 	}

t/t1092-sparse-checkout-compatibility.sh

@@ -376,4 +376,21 @@ test_expect_success 'clean' '
 	test_sparse_match test_path_is_dir folder1
 '
 
+test_expect_success 'submodule handling' '
+	init_repos &&
+
+	test_all_match mkdir modules &&
+	test_all_match touch modules/a &&
+	test_all_match git add modules &&
+	test_all_match git commit -m "add modules directory" &&
+
+	run_on_all git submodule add "$(pwd)/initial-repo" modules/sub &&
+	test_all_match git commit -m "add submodule" &&
+
+	# having a submodule prevents "modules" from collapse
+	test-tool -C sparse-index read-cache --table >cache &&
+	grep "100644 blob .*	modules/a" cache &&
+	grep "160000 commit $(git -C initial-repo rev-parse HEAD)	modules/sub" cache
+'
+
 test_done