chgserver: resolve relative path before sending via system channel The chgserver may have a different cwd from the client because of the side effect of "--cwd" and other possible os.chdir done by extensions. Therefore relative paths can be misunderstood by the client. This patch solves it by expanding relative cwd path to absolute one before sending them via the 'S' channel. It can help chg to pass a testcase in test-alias.t later.