only make 1 hardlink max between pointer file and annex object

If multiple files point to the same annex object, the user may want to
modify them independently, so don't use a hard link.

Also, check diskreserve when copying.

Command/Smudge.hs

@@ -86,7 +86,7 @@ ingest file = do
 		}
 	k <- fst . fromMaybe (error "failed to generate a key")
 		<$> genKey source backend
-	-- Hard link (or copy) file content to annex
+	-- Hard link (or copy) file content to annex object
 	-- to prevent it from being lost when git checks out
 	-- a branch not containing this file.
 	r <- linkAnnex k file