connect: split ssh command line options into separate function

connect: split ssh command line options into separate function

The git_connect function is growing long.  Split the portion that
discovers an ssh command and options it accepts before the service
name and path to a separate function to make it easier to read.

No functional change intended.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Reviewed-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>