Discover refs via smart HTTP server when available

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 <redacted>
CC: Daniel Barkalow <redacted>
Signed-off-by: Junio C Hamano <redacted>