revset-only: use __nonzero__ to check if a revset is empty For some smartsets, computing length is more expensive than checking if the set is empty.

_spanset: drop __getitem__ implementation It is expensive and not part of the official smartset API.

filteredset: drop __getitem__ implementation It is expensive and not part of the official smartset API.

generatorset: implement __len__ It was the only smartset class without a `__len__` implementation.

revset: make __len__ part of the offical API It is common for code to ask for the length of a revset. In fact, all but generatorset already implement it.

cmdserver: add service that listens on unix domain socket and forks process Typical use case of 'unix' mode is a background hg daemon. $ hg serve --cmdserver unix --cwd / -a /tmp/hg-`id -u`.sock Unlike 'pipe' mode in which parent process keeps stdio channel, 'unix' server can be detached. So clients can freely connect and disconnect from server, saving Python start-up time. It might be better to write "--cmdserver socket -a unix:/sockpath" instead of "--cmdserver unix -a /sockpath" in case hgweb gets the ability to listen on unix domain socket.

test-commandserver: add connector for unix domain socket server The next patch will introduce --cmdserver unix.

test-commandserver: allow check() to make connection in different way The next patch will add connector for 'unix' mode server.