Discover refs via smart HTTP server when available
Instead of loading the cached info/refs, try to use the smart HTTP
version when the server supports it. Since the smart variant is
actually the pkt-line stream from the start of either upload-pack
or receive-pack we need to parse these through get_remote_heads,
which requires a background thread to feed its pipe.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
CC: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>