Smart fetch and push over HTTP: server side

Smart fetch and push over HTTP: server side

Requests for $GIT_URL/git-receive-pack and $GIT_URL/git-upload-pack
are forwarded to the corresponding backend process by directly
executing it and leaving stdin and stdout connected to the invoking
web server.  Prior to starting the backend process the HTTP response
headers are sent, thereby freeing the backend from needing to know
about the HTTP protocol.

Requests that are encoded with Content-Encoding: gzip are
automatically inflated before being streamed into the backend.
This is primarily useful for the git-upload-pack backend, which
receives highly repetitive text data from clients that easily
compresses to 50% of its original size.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>