transport: add a hash algorithm member

transport: add a hash algorithm member

When connecting to a remote system, we need to know what hash algorithm
it will be using to talk to us.  Add a hash_algo member to struct
transport and add a function to read this data from the transport
object.

Signed-off-by: brian m. carlson <redacted>
Signed-off-by: Junio C Hamano <redacted>