message: Simplify json production and append to archive
  Per github review.
diff: use std::fs::{self, File};
  -use std::io::{Cursor, SeekFrom, Write};
  use std::io::prelude::*;
  +use std::io::SeekFrom;
  use std::path::{self, Path, PathBuf};
  use std::sync::Arc;
  @@ -370,16 +370,14 @@ fn tar(
  header.set_path(&path).chain_err(|| {
  format!("failed to add to archive: `{}`", fnd)
  })?;
  - let mut buff = Cursor::new(Vec::with_capacity(256));
  - writeln!(buff, "{}", serde_json::to_string_pretty(json)?)?;
  + let json = format!("{}\n", serde_json::to_string_pretty(json)?);
  let mut header = Header::new_ustar();
  header.set_path(&path)?;
  header.set_entry_type(EntryType::file());
  header.set_mode(0o644);
  - header.set_size(buff.position() as u64);
  + header.set_size(json.len() as u64);
  header.set_cksum();
  - buff.seek(SeekFrom::Start(0))?;
  - ar.append(&header, &mut buff).chain_err(|| {
  + ar.append(&header, json.as_bytes()).chain_err(|| {
  internal(format!("could not archive source file `{}`", fnd))
  })?;
  }
  