add gitAnnexTmpWorkDir and withTmpWorkDir

Needed to run youtube-dl in, but could also be useful for other stuff.

The tricky part of this was making the workdir be cleaned up whenever the
tmp object file is cleaned up.

This commit was sponsored by Ole-Morten Duesund on Patreon.

Command/DropUnused.hs

@@ -55,5 +55,5 @@ perform from numcopies key = case from of
 performOther :: (Key -> Git.Repo -> FilePath) -> Key -> CommandPerform
 performOther filespec key = do
 	f <- fromRepo $ filespec key
-	liftIO $ nukeFile f
+	pruneTmpWorkDirBefore f (liftIO . nukeFile)
 	next $ return True