connect: refactor git_connect to only get the protocol version once

connect: refactor git_connect to only get the protocol version once

Instead of having each builtin transport asking for which protocol
version the user has configured in 'protocol.version' by calling
`get_protocol_version_config()` multiple times, factor this logic out
so there is just a single call at the beginning of `git_connect()`.

This will be helpful in the next patch where we can have centralized
logic which determines if we need to request a different protocol
version than what the user has configured.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>