connect: Add the envvar GIT_SSH_VARIANT and ssh.variant config

connect: Add the envvar GIT_SSH_VARIANT and ssh.variant config

This environment variable and configuration value allow to
override the autodetection of plink/tortoiseplink in case that
Git gets it wrong.

[jes: wrapped overly-long lines, factored out and changed
get_ssh_variant() to handle_ssh_variant() to accomodate the
change from the putty/tortoiseplink variables to
port_option/needs_batch, adjusted the documentation, free()d
value obtained from the config.]

Signed-off-by: Segev Finer <segev208@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>