ssh: introduce a 'simple' ssh variant

When using the 'ssh' transport, the '-o' option is used to specify an
environment variable which should be set on the remote end.  This allows
git to send additional information when contacting the server,
requesting the use of a different protocol version via the
'GIT_PROTOCOL' environment variable like so: "-o SendEnv=GIT_PROTOCOL".

Unfortunately not all ssh variants support the sending of environment
variables to the remote end.  To account for this, only use the '-o'
option for ssh variants which are OpenSSH compliant.  This is done by
checking that the basename of the ssh command is 'ssh' or the ssh
variant is overridden to be 'ssh' (via the ssh.variant config).

Other options like '-p' and '-P', which are used to specify a specific
port to use, or '-4' and '-6', which are used to indicate that IPV4 or
IPV6 addresses should be used, may also not be supported by all ssh
variants.

Currently if an ssh command's basename wasn't 'plink' or
'tortoiseplink' git assumes that the command is an OpenSSH variant.
Since user configured ssh commands may not be OpenSSH compliant, tighten
this constraint and assume a variant of 'simple' if the basename of the
command doesn't match the variants known to git.  The new ssh variant
'simple' will only have the host and command to execute ([username@]host
command) passed as parameters to the ssh command.

Update the Documentation to better reflect the command-line options sent
to ssh commands based on their variant.

Reported-by: Jeffrey Yasskin <jyasskin@google.com>
Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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