transport: extract common fetch_pack() call

transport: extract common fetch_pack() call

In the switch statement, the difference between the `protocol_v2` and
`protocol_v{1,0}` arms is a preparatory call to die_if_server_options() in
the latter. The fetch_pack() call is identical in both arms. However,
since this fetch_pack() call has so many parameters, it is not
immediately obvious that the call is identical in both cases.

Extract the common fetch_pack() call out of the switch statement so that
code duplication is reduced and the logic is more clear for future
readers. While we're at it, rewrite the switch statement as an if-else
tower for increased clarity.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>