http-push: set algorithm when reading object ID

http-push: set algorithm when reading object ID

In most places in the codebase, we use oidread to properly read an
object ID into a struct object_id.  However, in the HTTP code, we end up
needing to parse a loose object path with a slash in it, so we can't do
that.  Let's instead explicitly set the algorithm in this function so we
can rely on it in the future.

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