Fix data loss bug in directory special remote

When moving a file to the remote failed, and partially transferred content
was left behind in the directory, re-running the same move would think it
succeeded and delete the local copy.

I reproduced data loss when moving files to a partition that was almost
full. Interrupting a transfer could have similar results.

Easily fixed by using a temp file which is then moved atomically into place
once the transfer completes.

I've audited other calls to copyFileExternal, and other special remote
file transfer code; everything else seems to use temp files correctly
(rsync, git), or otherwise use atomic transfers (bup, S3).

Remote/Directory.hs

@@ -98,11 +98,13 @@ storeEncrypted d (cipher, enck) k = do
 storeHelper :: FilePath -> Key -> (FilePath -> IO Bool) -> IO Bool
 storeHelper d key a = do
 	let dest = Prelude.head $ locations d key
+	let tmpdest = dest ++ ".tmp"
 	let dir = parentDir dest
 	createDirectoryIfMissing True dir
 	allowWrite dir
-	ok <- a dest
+	ok <- a tmpdest
 	when ok $ do
+		renameFile tmpdest dest
 		preventWrite dest
 		preventWrite dir
 	return ok