Allow cl_repo_get_bool to work with missing key

One of the test helpers provides a quick way for looking up a
boolean key.  But if the key way missing completely, the check
would actually raise an error.  Given the way we use this helper,
if the key is missing, this should just return false, I think.

tests/clar_libgit2.c

@@ -408,7 +408,8 @@ int cl_repo_get_bool(git_repository *repo, const char *cfg)
 	int val = 0;
 	git_config *config;
 	cl_git_pass(git_repository_config(&config, repo));
-	cl_git_pass(git_config_get_bool(&val, config, cfg));;
+	if (git_config_get_bool(&val, config, cfg) < 0)
+		giterr_clear();
 	git_config_free(config);
 	return val;
 }