Use hash for ControlPath instead of full variables

The generated socket path can be too long, if your FQDN is very long...

Typical error message from ssh client:
unix_listener: path "/tmp/ssh-fqduawon/master-USER@HOST:PORT.qfCZ51OAZgTzVLbg" too long for Unix domain socket

Use a hashed version instead, to keep within the socket file path limit.

This requires OpenSSH_6.7p1, or later.

Change-Id: Ia4bb9ae8aac6c4ee31d5a458f917f3753f40001b
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/255632
Reviewed-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Anders Björklund <anders.bjorklund.2@volvocars.com>