rsync special remote

Fully tested and working, including resuming and encryption. (Though not
resuming when sending *with* encryption; gpg doesn't produce identical
output each time.)

Uses same layout as the directory special remote and the .git/annex/objects/
directory.

Content.hs

@@ -12,6 +12,7 @@ module Content (
 	logStatusFor,
 	getViaTmp,
 	getViaTmpUnchecked,
+	withTmp,
 	checkDiskSpace,
 	preventWrite,
 	allowWrite,
@@ -127,6 +128,17 @@ getViaTmpUnchecked key action = do
 			-- to resume its transfer
 			return False
 
+{- Creates a temp file, runs an action on it, and cleans up the temp file. -}
+withTmp :: Key -> (FilePath -> Annex a) -> Annex a
+withTmp key action = do
+	g <- Annex.gitRepo
+	let tmp = gitAnnexTmpLocation g key
+	liftIO $ createDirectoryIfMissing True (parentDir tmp)
+	res <- action tmp
+	tmp_exists <- liftIO $ doesFileExist tmp
+	when tmp_exists $ liftIO $ removeFile tmp
+	return res
+
 {- Checks that there is disk space available to store a given key,
  - throwing an error if not. -}
 checkDiskSpace :: Key -> Annex ()