use bloom filter in second pass of sync --all --content

This is needed because when preferred content matches on files,
the second pass would otherwise want to drop all keys. Using a bloom filter
avoids this, and in the case of a false positive, a key will be left
undropped that preferred content would allow dropping. Chances of that
happening are a mere 1 in 1 million.

Annex/BloomFilter.hs

@@ -40,11 +40,11 @@ bloomBitsHashes = do
  - Once the action completes, the mutable filter is frozen
  - for later use.
  -}
-genBloomFilter :: Hashable v => ((v -> Annex ()) -> Annex b) -> Annex (Bloom v)
+genBloomFilter :: Hashable v => ((v -> Annex ()) -> Annex ()) -> Annex (Bloom v)
 genBloomFilter populate = do
 	(numbits, numhashes) <- bloomBitsHashes
 	bloom <- lift $ newMB (cheapHashes numhashes) numbits
-	_ <- populate $ \v -> lift $ insertMB bloom v
+	populate $ \v -> lift $ insertMB bloom v
 	lift $ unsafeFreezeMB bloom
   where
 	lift = liftIO . stToIO