fix test failure locking an unlocked not present file

In v5, that was not possible, but it is in v6, and so the test was failing.

Investigating, it turns out that locking was copying the pointer file
content to the annex object despite the content not being present. So,
add a check to prevent that.

Command/Lock.hs

@@ -46,7 +46,7 @@ startNew file key = ifM (isJust <$> isAnnexLink file)
 	)
   where
 	go (Just key')
-		| key' == key = cont False
+		| key' == key = error "content not present; cannot lock"
 		| otherwise = errorModified
 	go Nothing = 
 		ifM (isUnmodified key file)