use InodeCache to avoid races in import from directory special remote

This does not avoid all possible races, but it does avoid all likely
ones, and is demonstratably better than git's own handling of races
where files get modified at the same time as it's updating the working
tree.

The main thing this won't detect are not unlikely races where part
of a file gets changed while it's being copied and then the file is
restored to its original condition before the modification check.
No, it's more likely that the limitations of checking inode, size,
and mtime won't detect certian modifications, involving eg mmapped
files.

Annex/Import.hs

@@ -233,12 +233,14 @@ downloadImport remote importtreeconfig importablecontents = do
 		(k:_) -> return $ Just (loc, k)
 		[] -> checkDiskSpaceToGet tmpkey Nothing $
 			withTmp tmpkey $ \tmpfile ->
-				Remote.retrieveExportWithContentIdentifier ia loc cid tmpfile (ingestkey loc tmpfile) p >>= \case
-					Just k -> do
-						recordcidkey cidmap db cid k
-						logStatus k InfoPresent
-						logChange k (Remote.uuid remote) InfoPresent
-						return $ Just (loc, k)
+				Remote.retrieveExportWithContentIdentifier ia loc cid tmpfile (mkkey loc tmpfile) p >>= \case
+					Just k -> tryNonAsync (moveAnnex k tmpfile) >>= \case
+						Right True -> do
+							recordcidkey cidmap db cid k
+							logStatus k InfoPresent
+							logChange k (Remote.uuid remote) InfoPresent
+							return $ Just (loc, k)
+						_ -> return Nothing
 					Nothing -> return Nothing
 	  where
 		-- TODO progress bar
@@ -246,7 +248,7 @@ downloadImport remote importtreeconfig importablecontents = do
 		ia = Remote.importActions remote
 		tmpkey = importKey cid sz
 	
-	ingestkey loc tmpfile = do
+	mkkey loc tmpfile = do
 		f <- fromRepo $ fromTopFilePath $ locworktreefilename loc
 		backend <- chooseBackend f
 		let ks = KeySource
@@ -254,12 +256,7 @@ downloadImport remote importtreeconfig importablecontents = do
 			, contentLocation = tmpfile
 			, inodeCache = Nothing
 			}
-		genKey ks backend >>= \case
-			Nothing -> return Nothing
-			Just (k, _) ->
-				tryNonAsync (moveAnnex k tmpfile) >>= \case
-					Right True -> return (Just k)
-					_ -> return Nothing
+		fmap fst <$> genKey ks backend
 
 	locworktreefilename loc = asTopFilePath $ case importtreeconfig of
 		ImportTree -> fromImportLocation loc