Williams: Introducing Git protocol version 2

Brandon Williams writesabout the new Git remote protocol that will debut in the 2.18 release."We recently rolled out support for protocol version 2 at Google andhave seen a performance improvement of 3x for no-op fetches of a singlebranch on repositories containing 500k references. Protocol v2 has alsoenabled a reduction of 8x of the overhead bytes (non-packfile) sent fromgooglesource.com servers. A majority of this improvement is due tofiltering references advertised by the server to the refs the client hasexpressed interest in."