Flush the pack header as soon as its ready

Flush the pack header as soon as its ready

When the output stream is deeply buffered (e.g. 1 MiB or more in
an HTTP servlet on some containers) trying to kick out the header
earlier will prevent the client from stalling hard while the first
1 MiB is received and it can process the pack header.  Forcing a
flush here lets the client see the header and start its progress
monitor for "Receiving objects: (1/N)" so the user knows there
is still activity occurring, even though the buffering may cause
there to be some lag as the buffer fills up on the sending side.

Change-Id: I3edf39e8f703fe87a738dc236d426b194db85e3a
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>