ssh: 'simple' variant does not support --port

ssh: 'simple' variant does not support --port

When trying to connect to an ssh:// URL with port explicitly specified
and the ssh command configured with GIT_SSH does not support such a
setting, it is less confusing to error out than to silently suppress
the port setting and continue.

This requires updating the GIT_SSH setting in t5603-clone-dirname.sh.
That test is about the directory name produced when cloning various
URLs.  It uses an ssh wrapper that ignores all its arguments but does
not declare that it supports a port argument; update it to set
GIT_SSH_VARIANT=ssh to do so.  (Real-life ssh wrappers that pass a
port argument to OpenSSH would also support -G and would not require
such an update.)

Reported-by: William Yan <wyan@google.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>