Merge branch 'jn/ssh-wrappers'

The ssh-variant 'simple' introduced earlier broke existing
installations by not passing --port/-4/-6 and not diagnosing an
attempt to pass these as an error.  Instead, default to
automatically detect how compatible the GIT_SSH/GIT_SSH_COMMAND is
to OpenSSH convention and then error out an invocation to make it
easier to diagnose connection errors.

* jn/ssh-wrappers:
  connect: correct style of C-style comment
  ssh: 'simple' variant does not support --port
  ssh: 'simple' variant does not support -4/-6
  ssh: 'auto' variant to select between 'ssh' and 'simple'
  connect: split ssh option computation to its own function
  connect: split ssh command line options into separate function
  connect: split git:// setup into a separate function
  connect: move no_fork fallback to git_tcp_connect
  ssh test: make copy_ssh_wrapper_as clean up after itself

Author: gitster

Documentation/config.txt

@@ -2108,16 +2108,22 @@ matched against are those given directly to Git commands.  This means any URLs
 visited as a result of a redirection do not participate in matching.
 
 ssh.variant::
-	Depending on the value of the environment variables `GIT_SSH` or
-	`GIT_SSH_COMMAND`, or the config setting `core.sshCommand`, Git
-	auto-detects whether to adjust its command-line parameters for use
-	with ssh (OpenSSH), plink or tortoiseplink, as opposed to the default
-	(simple).
-+
-The config variable `ssh.variant` can be set to override this auto-detection;
-valid values are `ssh`, `simple`, `plink`, `putty` or `tortoiseplink`. Any
-other value will be treated as normal ssh. This setting can be overridden via
-the environment variable `GIT_SSH_VARIANT`.
+	By default, Git determines the command line arguments to use
+	based on the basename of the configured SSH command (configured
+	using the environment variable `GIT_SSH` or `GIT_SSH_COMMAND` or
+	the config setting `core.sshCommand`). If the basename is
+	unrecognized, Git will attempt to detect support of OpenSSH
+	options by first invoking the configured SSH command with the
+	`-G` (print configuration) option and will subsequently use
+	OpenSSH options (if that is successful) or no options besides
+	the host and remote command (if it fails).
++
+The config variable `ssh.variant` can be set to override this detection.
+Valid values are `ssh` (to use OpenSSH options), `plink`, `putty`,
+`tortoiseplink`, `simple` (no options except the host and remote command).
+The default auto-detection can be explicitly requested using the value
+`auto`.  Any other value is treated as `ssh`.  This setting can also be
+overridden via the environment variable `GIT_SSH_VARIANT`.
 +
 The current command-line parameters used for each variant are as
 follows: