ssh: 'auto' variant to select between 'ssh' and 'simple'

ssh: 'auto' variant to select between 'ssh' and 'simple'

Android's "repo" tool is a tool for managing a large codebase
consisting of multiple smaller repositories, similar to Git's
submodule feature.  Starting with Git 94b8ae5a (ssh: introduce a
'simple' ssh variant, 2017-10-16), users noticed that it stopped
handling the port in ssh:// URLs.

The cause: when it encounters ssh:// URLs, repo pre-connects to the
server and sets GIT_SSH to a helper ".repo/repo/git_ssh" that reuses
that connection.  Before 94b8ae5a, the helper was assumed to support
OpenSSH options for lack of a better guess and got passed a -p option
to set the port.  After that patch, it uses the new default of a
simple helper that does not accept an option to set the port.

The next release of "repo" will set GIT_SSH_VARIANT to "ssh" to avoid
that.  But users of old versions and of other similar GIT_SSH
implementations would not get the benefit of that fix.

So update the default to use OpenSSH options again, with a twist.  As
observed in 94b8ae5a, we cannot assume that $GIT_SSH always handles
OpenSSH options: common helpers such as travis-ci's dpl[*] are
configured using GIT_SSH and do not accept OpenSSH options.  So make
the default a new variant "auto", with the following behavior:

 1. First, check for a recognized basename, like today.

 2. If the basename is not recognized, check whether $GIT_SSH supports
    OpenSSH options by running

	$GIT_SSH -G <options> <host>

    This returns status 0 and prints configuration in OpenSSH if it
    recognizes all <options> and returns status 255 if it encounters
    an unrecognized option.  A wrapper script like

	exec ssh -- "$@"

    would fail with

	ssh: Could not resolve hostname -g: Name or service not known

    , correctly reflecting that it does not support OpenSSH options.
    The command is run with stdin, stdout, and stderr redirected to
    /dev/null so even a command that expects a terminal would exit
    immediately.

 3. Based on the result from step (2), behave like "ssh" (if it
    succeeded) or "simple" (if it failed).

This way, the default ssh variant for unrecognized commands can handle
both the repo and dpl cases as intended.

This autodetection has been running on Google workstations since
2017-10-23 with no reported negative effects.

[*] https://github.com/travis-ci/dpl/blob/6c3fddfda1f2a85944c544446b068bac0a77c049/lib/dpl/provider.rb#L215

Reported-by: William Yan <wyan@google.com>
Improved-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>