Changes

Buffer the response until request parsing has done

This is a continuation from https://git.eclipse.org/r/#/c/4716/. For a
non-bidirectional request, we need to consume the request before writing
any response. In UploadPack, we write "shallow"/"unshallow" responses
before parsing "have" lines. This has happened not to be a problem most
of the time in the smart HTTP protocol because the underlying
InputStream has a 32 KiB buffer in SmartOutputStream.

Change-Id: I7c61659e7c4e8bd49a8b17e2fe9be67bb32933d3
Signed-off-by: Masaya Suzuki <masayasuzuki@google.com>