external backends wip

It's able to start them up, the only thing not implemented is generating
and verifying keys. And, the key translation for HasExt.

Command/Fsck.hs

@@ -199,7 +199,7 @@ performRemote key afile backend numcopies remote =
 
 startKey :: Maybe Remote -> Incremental -> (Key, ActionItem) -> NumCopies -> CommandStart
 startKey from inc (key, ai) numcopies =
-	case Backend.maybeLookupBackendVariety (fromKey keyVariety key) of
+	Backend.maybeLookupBackendVariety (fromKey keyVariety key) >>= \case
 		Nothing -> stop
 		Just backend -> runFsck inc ai key $
 			case from of
@@ -261,7 +261,7 @@ verifyLocationLog key keystatus ai = do
 	 - insecure hash is present. This should only be able to happen
 	 - if the repository already contained the content before the
 	 - config was set. -}
-	when (present && not (Backend.isCryptographicallySecure key)) $
+	whenM (pure present <&&> (not <$> Backend.isCryptographicallySecure key)) $
 		whenM (annexSecureHashesOnly <$> Annex.getGitConfig) $
 			warning $ "** Despite annex.securehashesonly being set, " ++ obj ++ " has content present in the annex using an insecure " ++ decodeBS (formatKeyVariety (fromKey keyVariety key)) ++ " key"
 

Command/Smudge.hs

@@ -131,7 +131,10 @@ clean file = do
 		-- Look up the backend that was used for this file
 		-- before, so that when git re-cleans a file its
 		-- backend does not change.
-		let oldbackend = maybe Nothing (maybeLookupBackendVariety . fromKey keyVariety) oldkey
+		oldbackend <- maybe
+			(pure Nothing)
+			(maybeLookupBackendVariety . fromKey keyVariety)
+			oldkey
 		-- Can't restage associated files because git add
 		-- runs this and has the index locked.
 		let norestage = Restage False

Command/TestRemote.hs

@@ -288,7 +288,7 @@ test runannex mkr mkk =
 			Nothing -> return True
 		runannex a' @? "failed"
 	present r k b = (== Right b) <$> Remote.hasKey r k
-	fsck _ k = case maybeLookupBackendVariety (fromKey keyVariety k) of
+	fsck _ k = maybeLookupBackendVariety (fromKey keyVariety k) >>= \case
 		Nothing -> return True
 		Just b -> case Types.Backend.verifyKeyContent b of
 			Nothing -> return True

Command/Version.hs

@@ -59,7 +59,8 @@ showPackageVersion = do
 	vinfo "build flags" $ unwords buildFlags
 	vinfo "dependency versions" $ unwords dependencyVersions
 	vinfo "key/value backends" $ unwords $
-		map (decodeBS . formatKeyVariety . B.backendVariety) Backend.list
+		map (decodeBS . formatKeyVariety . B.backendVariety) Backend.builtinList
+		++ ["X*"]
 	vinfo "remote types" $ unwords $ map R.typename Remote.remoteTypes
 	vinfo "operating system" $ unwords [os, arch]
 	vinfo "supported repository versions" $