UploadPack: Avoid unnecessary flush in smart HTTP

UploadPack: Avoid unnecessary flush in smart HTTP

Under smart HTTP the biDirectionalPipe flag is false, and we return
back immediately at this point in the negotiation process.  There is
no need to flush the stream to the client, the request is over and
it will be automatically flushed out by the higher level servlet
that invoked us.  Avoiding flush here allows us to only use flush
after a progress message is sent during pack generation.

Change-Id: Id0c8b7e95e3be6ca4c1b479e096bed6b0283b828
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>