chgserver: add utilities to calculate confighash confighash is the hash of sensitive config items like [extensions], and sensitive environment variables like HG*, LD_*, etc. The config items can come from global, user, repo config, and command line flags. For chgserver, it is designed that once confighash changes, the server is not qualified to serve its client and should redirect the client to a new server. The server does not need to exit in this case, since it can still be valid (have a matched confighash) to serve other chg clients.