### Please describe the problem.

The latest released version of git-annex (6.20161031) breaks on all platforms that do not have ssh 7.3 installed as it relies on "Include" ssh_config(5) configuration flag that appeared in 7.3p1. The broken platforms include macOS Sierra and Ubuntu 16.04 Xenial (LTS) both include ssh 7.2

    # Ubuntu 16.04
    $ ssh -V
    OpenSSH_7.2p2 Ubuntu-4ubuntu2.1, OpenSSL 1.0.2g  1 Mar 2016

    # macOS
    $ ssh -V
    OpenSSH_7.2p2, LibreSSL 2.4.1

Here is what it looks like:

    $ git annex move --to vir
    move Foobar.mkv (checking vir...) .git/annex/ssh.config: line 1: Bad configuration option: include
    .git/annex/ssh.config: line 2: Bad configuration option: include
    .git/annex/ssh.config: terminating, 2 bad configuration options
    (unable to check vir
    CallStack (from HasCallStack):
      error, called at ./Remote/Helper/Messages.hs:32:15 in main:Remote.Helper.Messages) failed


    $ cat .git/annex/ssh.config
    Include ~/.ssh/config
    Include /etc/ssh/ssh_config
    ServerAliveInterval 60

> This is already fixed in git. [[done]] --[[Joey]]
