Make lock and unlock work in v6 repos on files whose content is not present.

Command/Lock.hs

@@ -45,7 +45,7 @@ startNew file key = ifM (isJust <$> isAnnexLink file)
 	)
   where
 	go (Just key')
-		| key' == key = error "content not present; cannot lock"
+		| key' == key = cont True
 		| otherwise = errorModified
 	go Nothing = 
 		ifM (isUnmodified key file)