Smart fetch over HTTP: client side

Smart fetch over HTTP: client side

The git-remote-curl backend detects if the remote server supports
the git-upload-pack service, and if so, runs git-fetch-pack locally
in a pipe to generate the want/have commands.

The advertisements from the server that were obtained during the
discovery are passed into git-fetch-pack before the POST request
starts, permitting server capability discovery and enablement.

Common objects that are discovered are appended onto the request as
have lines and are sent again on the next request.  This allows the
remote side to reinitialize its in-memory list of common objects
during the next request.

Because all requests are relatively short, below git-remote-curl's
1 MiB buffer limit, requests will use the standard Content-Length
header and be valid HTTP/1.0 POST requests.  This makes the fetch
client more tolerant of proxy servers which don't support HTTP/1.1
or the chunked transfer encoding.

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