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

diff --cc Documentation/config.txt
Simple merge

diff --cc upload-pack.c
Simple merge