submodule: add failing test for backslash in url

tests/submodule/lookup.c

@@ -151,6 +151,29 @@ void test_submodule_lookup__lookup_even_with_missing_index(void)
 	test_submodule_lookup__simple_lookup(); /* baseline should still pass */
 }
 
+void test_submodule_lookup__backslashes(void)
+{
+	git_config *cfg;
+	git_submodule *sm;
+	git_repository *subrepo;
+	git_buf buf = GIT_BUF_INIT;
+	const char *backslashed_path = "..\\submod2_target";
+
+	cl_git_pass(git_config_open_ondisk(&cfg, "submod2/.gitmodules"));
+	cl_git_pass(git_config_set_string(cfg, "submodule.sm_unchanged.url", backslashed_path));
+	git_config_free(cfg);
+
+	cl_git_pass(git_submodule_lookup(&sm, g_repo, "sm_unchanged"));
+	cl_assert_equal_s(backslashed_path, git_submodule_url(sm));
+	cl_git_pass(git_submodule_open(&subrepo, sm));
+
+	cl_git_pass(git_submodule_resolve_url(&buf, g_repo, backslashed_path));
+
+	git_buf_free(&buf);
+	git_submodule_free(sm);
+	git_repository_free(subrepo);
+}
+
 static void baseline_tests(void)
 {
 	/* small baseline that should work even if we change the index or make