close the git add race

There's a race adding a new file to the annex: The file is moved to the
annex and replaced with a symlink, and then we git add the symlink. If
someone comes along in the meantime and replaces the symlink with
something else, such as a new large file, we add that instead. Which could
be bad..

This race is fixed by avoiding using git add, instead the symlink is
directly staged into the index.

It would be nice to make `git annex add` use this same technique.
I have not done so yet because it currently runs git update-index once per
file, which would slow does `git annex add`. A future enhancement would be
to extend the Git.Queue to include the ability to run update-index with
a list of Streamers.

Command/Add.hs

@@ -94,8 +94,9 @@ undo file key e = do
 			src <- inRepo $ gitAnnexLocation key
 			liftIO $ moveFile src file
 
-{- Creates the symlink to the annexed content. -}
-link :: FilePath -> Key -> Bool -> Annex ()
+{- Creates the symlink to the annexed content, and also returns the link's
+ - text. -}
+link :: FilePath -> Key -> Bool -> Annex FilePath
 link file key hascontent = handle (undo file key) $ do
 	l <- calcGitLink file key
 	liftIO $ createSymbolicLink l file
@@ -109,11 +110,13 @@ link file key hascontent = handle (undo file key) $ do
 			mtime <- modificationTime <$> getFileStatus file
 			touch file (TimeSpec mtime) False
 
+	return l
+
 {- Note: Several other commands call this, and expect it to 
  - create the symlink and add it. -}
 cleanup :: FilePath -> Key -> Bool -> CommandCleanup
 cleanup file key hascontent = do
-	link file key hascontent
+	_ <- link file key hascontent
 	params <- ifM (Annex.getState Annex.force)
 		( return [Param "-f"]
 		, return []