git_reference_next_name must match git_reference_next

The git_reference_next API silently skips invalid references when
scanning the loose refs.  The git_reference_next_name API should
skip the same ones even though it isn't creating the reference
object.

This adds a test with a an invalid loose reference and makes sure
that both APIs skip the same entries and generate the same results.

src/refdb_fs.c

@@ -452,6 +452,9 @@ static int loose_lookup(
 	git_buf ref_file = GIT_BUF_INIT;
 	int error = 0;
 
+	if (out)
+		*out = NULL;
+
 	error = reference_read(&ref_file, NULL, backend->path, ref_name, NULL);
 
 	if (error < 0)
@@ -465,15 +468,17 @@ static int loose_lookup(
 			goto done;
 		}
 
-		*out = git_reference__alloc_symbolic(ref_name, target);
+		if (out)
+			*out = git_reference__alloc_symbolic(ref_name, target);
 	} else {
 		if ((error = loose_parse_oid(&oid, ref_name, &ref_file)) < 0)
 			goto done;
 
-		*out = git_reference__alloc(ref_name, &oid, NULL);
+		if (out)
+			*out = git_reference__alloc(ref_name, &oid, NULL);
 	}
 
-	if (*out == NULL)
+	if (out && *out == NULL)
 		error = -1;
 
 done:
@@ -679,6 +684,11 @@ static int refdb_fs_backend__iterator_next_name(
 		if (git_strmap_exists(packfile, path))
 			continue;
 
+		if (loose_lookup(NULL, backend, path) != 0) {
+			giterr_clear();
+			continue;
+		}
+
 		*out = path;
 		return 0;
 	}

tests-clar/revwalk/basic.c

@@ -142,6 +142,28 @@ void test_revwalk_basic__glob_heads(void)
 	cl_assert(i == 14);
 }
 
+void test_revwalk_basic__glob_heads_with_invalid(void)
+{
+	int i;
+	git_oid oid;
+
+	test_revwalk_basic__cleanup();
+
+	_repo = cl_git_sandbox_init("testrepo");
+	cl_git_mkfile("testrepo/.git/refs/heads/garbage", "not-a-ref");
+
+	cl_git_pass(git_revwalk_new(&_walk, _repo));
+	cl_git_pass(git_revwalk_push_glob(_walk, "heads"));
+
+	for (i = 0; !git_revwalk_next(&oid, _walk); ++i)
+		/* walking */;
+
+	/* git log --branches --oneline | wc -l => 16 */
+	cl_assert_equal_i(16, i);
+
+	cl_fixture_cleanup("testrepo");
+}
+
 void test_revwalk_basic__push_head(void)
 {
 	int i = 0;