Merge branch 'jk/upload-pack-keepalive'

When running "fetch -q", a long silence while the sender side computes the set of objects to send can be mistaken by proxies as dropped connection. The server side has been taught to send a small empty messages to keep the connection alive. * jk/upload-pack-keepalive: upload-pack: bump keepalive default to 5 seconds upload-pack: send keepalive packets during pack computation