bundle2: add a comment about addchangegroup source and url

pull: call the `b2x-transactionclose` hook when closing the transaction We need a wider set of hooks to process all the changes that happened during the pull transaction. We reuse the experimental `b2x-transactionclose` hook set from server's unbundle for consistency. This hook is experimental and will not remains as-is forever, but this will open the door for experimentation in 3.2.

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.