Test config name validation

This is @nulltoken's work to test various invalid config section
and key names and make sure we are validating properly.

tests-clar/config/validkeyname.c

+#include "clar_libgit2.h"
+
+#include "config.h"
+
+static git_config *cfg;
+static const char *value;
+
+void test_config_validkeyname__initialize(void)
+{
+	cl_fixture_sandbox("config/config10");
+
+	cl_git_pass(git_config_open_ondisk(&cfg, "config10"));
+}
+
+void test_config_validkeyname__cleanup(void)
+{
+	git_config_free(cfg);
+	cfg = NULL;
+
+	cl_fixture_cleanup("config10");
+}
+
+static void assert_invalid_config_key_name(const char *name)
+{
+	cl_git_fail_with(git_config_get_string(&value, cfg, name),
+		GIT_EINVALIDSPEC);
+	cl_git_fail_with(git_config_set_string(cfg, name, "42"),
+		GIT_EINVALIDSPEC);
+	cl_git_fail_with(git_config_delete_entry(cfg, name),
+		GIT_EINVALIDSPEC);
+	cl_git_fail_with(git_config_get_multivar(cfg, name, "*", NULL, NULL),
+		GIT_EINVALIDSPEC);
+	cl_git_fail_with(git_config_set_multivar(cfg, name, "*", "42"),
+		GIT_EINVALIDSPEC);
+}
+
+void test_config_validkeyname__accessing_requires_a_valid_name(void)
+{
+	assert_invalid_config_key_name("");
+	assert_invalid_config_key_name(".");
+	assert_invalid_config_key_name("..");
+	assert_invalid_config_key_name("core.");
+	assert_invalid_config_key_name("d#ff.dirstat.lines");
+	assert_invalid_config_key_name("diff.dirstat.lines#");
+	assert_invalid_config_key_name("dif\nf.dirstat.lines");
+	assert_invalid_config_key_name("dif.dir\nstat.lines");
+	assert_invalid_config_key_name("dif.dirstat.li\nes");
+}
+
+static void assert_invalid_config_section_name(git_repository *repo, const char *name)
+{
+	cl_git_fail_with(git_config_rename_section(repo, "branch.remoteless", name), GIT_EINVALIDSPEC);
+}
+
+void test_config_validkeyname__renaming_a_section_requires_a_valid_name(void)
+{
+	git_repository *repo;
+
+	cl_git_pass(git_repository_open(&repo, cl_fixture("testrepo.git")));
+
+	assert_invalid_config_section_name(repo, "");
+	assert_invalid_config_section_name(repo, "bra\nch");
+	assert_invalid_config_section_name(repo, "branc#");
+	assert_invalid_config_section_name(repo, "bra\nch.duh");
+	assert_invalid_config_section_name(repo, "branc#.duh");
+
+	git_repository_free(repo);
+}