Support GIT_SSH and GIT_SSH_COMMAND

They are handled close the same as they are by git. However, unlike git,
git-annex sometimes needs to pass the -n parameter when using these.

So, this has the potential for breaking some setup, and perhaps there ought
to be a ANNEX_USE_GIT_SSH=1 needed to use these. But I'd rather avoid that
if possible, so let's see if anyone complains.

Almost all places where "ssh" was run have been changed to support the env
vars. Anything still calling sshOptions does not support them. In
particular, rsync special remotes don't. Seems that annex-rsync-transport
already gives sufficient control there.

(Fixed in passing: Remote.Helper.Ssh.toRepo used to extract
remoteAnnexSshOptions and pass them to sshOptions, which was redundant
since sshOptions also extracts those.)

This commit was sponsored by Jeff Goeke-Smith on Patreon.

doc/forum/GIT__95__SSH/comment_1_c2e827eeac7524845cad42671d77af95._comment

@@ -0,0 +1,14 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 1"""
+ date="2017-03-17T17:26:56Z"
+ content="""
+I've added support for this now.
+
+However, since git's interface to this doesn't let any options other than
+-p be passed to the command run by this, git-annex can't either. Which
+means it can't pass options for ssh connection caching. So, I'm afraid that
+using `GIT_SSH` will slow things down somewhat. Of course, you can always
+enable ssh connection caching yourself in either the `GIT_SSH` script or
+the ssh configuration.
+"""]]