avoid ingest lockdown file escaping the withOtherTmp call

Fixes bug that caused git-annex to fail to add a file when another
git-annex process cleaned up the temp directory it was using.

Solution is just to push withOtherTmp out to a higher level, so that
the whole ingest process can be completed inside it.

But in the assistant, that was not practical to do, since withOtherTmp runs
in the Annex monad and the assistant does not. Worked around by introducing
a separate temp directory that only the assistant uses for lockdown.
Since only one assistant can run at a time, it's easy to clean up that
directory of old cruft at startup.

Command/Add.hs

@@ -19,6 +19,7 @@ import Config
 import Annex.FileMatcher
 import Annex.Link
 import Annex.Version
+import Annex.Tmp
 import Git.FilePath
 
 cmd :: Command
@@ -137,11 +138,11 @@ start file = do
 		next $ next $ addFile file
 
 perform :: FilePath -> CommandPerform
-perform file = do
+perform file = withOtherTmp $ \tmpdir -> do
 	lockingfile <- not <$> addUnlocked
 	let cfg = LockDownConfig
 		{ lockingFile = lockingfile
-		, hardlinkFileTmp = True
+		, hardlinkFileTmpDir = Just tmpdir
 		}
 	lockDown cfg file >>= ingestAdd >>= finish
   where