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.

Remote/Helper/Ssh.hs

@@ -23,15 +23,10 @@ import Types.Remote
 import Types.Transfer
 import Config
 
-{- Generates parameters to ssh to a repository's host and run a command.
- - Caller is responsible for doing any neccessary shellEscaping of the
- - passed command. -}
-toRepo :: ConsumeStdin -> Git.Repo -> RemoteGitConfig -> [CommandParam] -> Annex [CommandParam]
-toRepo cs r gc sshcmd = do
-	let opts = map Param $ remoteAnnexSshOptions gc
+toRepo :: ConsumeStdin -> Git.Repo -> RemoteGitConfig -> SshCommand -> Annex (FilePath, [CommandParam])
+toRepo cs r gc remotecmd = do
 	let host = fromMaybe (giveup "bad ssh url") $ Git.Url.hostuser r
-	params <- sshOptions cs (host, Git.Url.port r) gc opts
-	return $ params ++ Param host : sshcmd
+	sshCommand cs (host, Git.Url.port r) gc remotecmd
 
 {- Generates parameters to run a git-annex-shell command on a remote
  - repository. -}
@@ -49,8 +44,7 @@ git_annex_shell cs r command params fields
 				: map shellEscape (toCommand shellopts) ++
 			uuidcheck u ++
 			map shellEscape (toCommand fieldopts)
-		sshparams <- toRepo cs r gc [Param sshcmd]
-		return $ Just ("ssh", sshparams)
+		Just <$> toRepo cs r gc sshcmd
 	| otherwise = return Nothing
   where
 	dir = Git.repoPath r