The command line options for ssh invocation needs to be tweaked for
some implementations of SSH (e.g. PuTTY plink wants "-P <port>"
while OpenSSH wants "-p <port>" to specify port to connect to), and
the variant was guessed when GIT_SSH environment variable is used
to specify it.  The logic to guess now applies to the command
specified by the newer GIT_SSH_COMMAND and also core.sshcommand
configuration variable, and comes with an escape hatch for users to
deal with misdetected cases.

* sf/putty-w-args:
  connect.c: stop conflating ssh command names and overrides
  connect: Add the envvar GIT_SSH_VARIANT and ssh.variant config
  git_connect(): factor out SSH variant handling
  connect: rename tortoiseplink and putty variables
  connect: handle putty/plink also in GIT_SSH_COMMAND