util: introduce a bufferedinputpipe utility To restore real time server output through ssh, we need to using polling feature (like select) on the pipes used to communicate with the ssh client. However we cannot use select alongside python level buffering of these pipe (because we need to know if the buffer is non-empty before calling select). However, unbuffered performance are terrible, presumably because the 'readline' call is issuing 'read(1)' call until it find a '\n'. To work around that we introduces our own overlay that do buffering by hand, exposing the state of the buffer to the outside world. The usage of polling IO will be introduced later in the 'sshpeer' module. All its logic will be very specific to the way mercurial communicate over ssh and does not belong to the generic 'util' module.