message: remove need for HEAD tag, this tag is not used
diff: //!
  use super::super::{Storage, StorageConfig, block_location, block_read, block_read_location, header_to_blockhash, packreader_init};
  -use super::super::{tag, blob};
  +use super::super::{blob};
  use super::super::epoch::{epoch_read_pack, epoch_open_packref};
  use super::super::containers::packfile;
  use super::super::types::{BlockHash, PackHash};
  @@ -271,14 +271,6 @@ impl<'a> ReverseIter<'a> {
  };
  Ok(ri)
  }
  -
  - pub fn new(storage: &'a Storage) -> Result<Self> {
  - let hh_bytes = match tag::read_hash(&storage, &tag::HEAD) {
  - None => return Err(Error::NoTagHead),
  - Some(t) => t
  - };
  - Self::from(storage, &hh_bytes)
  - }
  }
  impl<'a> iter::Iterator for ReverseIter<'a> {
  type Item = Block;
  