ssh exit status 255 is a connection problem

Previously, when the git config was unable to be read from a ssh remote,
it would try to git fetch from it to determine if the remote was
otherwise accessible. That was unnessary work, since exit status 255
indicates a connection problem.

As well as avoiding the extra work of the fetch, this also improves
things when a ssh remote cannot be connected to due to a problem with
the git-annex ssh control socket. In that situation, ssh will also exit 255.
Before, the git fetch was tried in that situation, and would succeed, since
it does not use the git-annex ssh control socket. git-annex would conclude
that git-annex-shell was not installed on the remote, which could be wrong.

I suppose it also used to be possible for the user to need to enter a
ssh password on each connection to the remote. If they entered the wrong
password for the git-annex-shell call, but then the right password for
the git fetch, it would also incorrectly set annex-ignore, and that
situation is also now fixed.

Remote/Git.hs

@@ -278,14 +278,25 @@ tryGitConfigRead autoinit r hasuuid
 	| Git.repoIsSsh r = storeUpdatedRemote $ do
 		v <- Ssh.onRemote NoConsumeStdin r
 			( pipedconfig Git.Config.ConfigList autoinit (Git.repoDescribe r)
-			, return (Left "configlist failed")
+			, error "internal"
 			)
 			"configlist" [] configlistfields
 		case v of
 			Right r'
 				| haveconfig r' -> return r'
-				| otherwise -> configlist_failed
-			Left _ -> configlist_failed
+				| otherwise -> do
+					configlist_failed
+					return r
+			Left exitcode -> do
+				-- ssh exits 255 when there was an error
+				-- connecting to the remote server.
+				if exitcode /= ExitFailure 255 
+					then do
+						configlist_failed
+						return r
+					else do
+						warning $ UnquotedString $ "Unable to connect to repository " ++ Git.repoDescribe r ++ " to get its annex.uuid configuration."
+						return r
 	| Git.repoIsHttp r = storeUpdatedRemote geturlconfig
 	| Git.GCrypt.isEncrypted r = handlegcrypt =<< getConfigMaybe (remoteAnnexConfig r "uuid")
 	| Git.repoIsUrl r = do
@@ -298,31 +309,35 @@ tryGitConfigRead autoinit r hasuuid
 	haveconfig = not . M.null . Git.config
 
 	pipedconfig st mustincludeuuuid configloc cmd params = do
-		v <- liftIO $ Git.Config.fromPipe r cmd params st
-		case v of
-			Right (r', val, _err) -> do
+		(r', val, exitcode, _err) <- liftIO $ 
+			Git.Config.fromPipe r cmd params st
+		if exitcode == ExitSuccess
+			then do
 				unless (isUUIDConfigured r' || val == mempty || not mustincludeuuuid) $ do
 					warning $ UnquotedString $ "Failed to get annex.uuid configuration of repository " ++ Git.repoDescribe r
 					warning $ UnquotedString $ "Instead, got: " ++ show val
 					warning "This is unexpected; please check the network transport!"
 				return $ Right r'
-			Left l -> do
+			else do
 				warning $ UnquotedString $ "Unable to parse git config from " ++ configloc
-				return $ Left (show l)
+				return $ Left exitcode
 
 	geturlconfig = Url.withUrlOptionsPromptingCreds $ \uo -> do
 		let url = Git.repoLocation r ++ "/config"
 		v <- withTmpFile "git-annex.tmp" $ \tmpfile h -> do
 			liftIO $ hClose h
 			Url.download' nullMeterUpdate Nothing url tmpfile uo >>= \case
-				Right () -> pipedconfig Git.Config.ConfigNullList
-					False url "git"
-					[ Param "config"
-					, Param "--null"
-					, Param "--list"
-					, Param "--file"
-					, File tmpfile
-					]
+				Right () ->
+					pipedconfig Git.Config.ConfigNullList
+						False url "git"
+						[ Param "config"
+						, Param "--null"
+						, Param "--list"
+						, Param "--file"
+						, File tmpfile
+						] >>= return . \case
+							Right r' -> Right r'
+							Left exitcode -> Left $ "git config exited " ++ show exitcode
 				Left err -> return (Left err)
 		case v of
 			Right r' -> do
@@ -342,15 +357,7 @@ tryGitConfigRead autoinit r hasuuid
 				warning $ UnquotedString $ url ++ " " ++ err
 				return r
 
-	{- Is this remote just not available, or does
-	 - it not have git-annex-shell?
-	 - Find out by trying to fetch from the remote. -}
-	configlist_failed = case Git.remoteName r of
-		Nothing -> return r
-		Just n -> do
-			whenM (inRepo $ Git.Command.runBool [Param "fetch", Param "--quiet", Param n]) $ do
-				set_ignore "does not have git-annex installed" True
-			return r
+	configlist_failed = set_ignore "does not have git-annex installed" True
 	
 	set_ignore msg longmessage = do
 		case Git.remoteName r of