connect: detect algorithm when fetching refs

connect: detect algorithm when fetching refs

If we're fetching refs, detect the hash algorithm and parse the refs
using that algorithm.

As mentioned in the documentation, if multiple versions of the
object-format capability are provided, we use the first.  No known
implementation supports multiple algorithms now, but they may in the
future.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>