connect: split ssh option computation to its own function

connect: split ssh option computation to its own function

This puts the determination of options to pass to each ssh variant
(see ssh.variant in git-config(1)) in one place.

A follow-up patch will use this in an initial dry run to detect which
variant to use when the ssh command is ambiguous.

No functional change intended yet.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>