avoid the dashed ssh hostname class of security holes

Security fix: Disallow hostname starting with a dash, which would get
passed to ssh and be treated an option. This could be used by an attacker
who provides a crafted ssh url (for eg a git remote) to execute arbitrary
code via ssh -oProxyCommand.

No CVE has yet been assigned for this hole.
The same class of security hole recently affected git itself,
CVE-2017-1000117.

Method: Identified all places where ssh is run, by git grep '"ssh"'
Converted them all to use a SshHost, if they did not already, for
specifying the hostname.

SshHost was made a data type with a smart constructor, which rejects
hostnames starting with '-'.

Note that git-annex already contains extensive use of Utility.SafeCommand,
which fixes a similar class of problem where a filename starting with a
dash gets passed to a program which treats it as an option.

This commit was sponsored by Jochen Bartl on Patreon.

Remote/Rsync.hs

@@ -38,6 +38,7 @@ import Types.Transfer
 import Types.Creds
 import Annex.DirHashes
 import Utility.Tmp
+import Utility.SshHost
 
 import qualified Data.Map as M
 
@@ -120,7 +121,8 @@ rsyncTransport gc url
 		case fromNull ["ssh"] (remoteAnnexRsyncTransport gc) of
 			"ssh":sshopts -> do
 				let (port, sshopts') = sshReadPort sshopts
-				    userhost = takeWhile (/=':') url
+				    userhost = either error id $ mkSshHost $ 
+				    	takeWhile (/= ':') url
 				(Param "ssh":) <$> sshOptions ConsumeStdin
 					(userhost, port) gc
 					(map Param $ loginopt ++ sshopts')