message: Buffer file writing for dep-info
  Use BufWriter when generating .d files with dep-info, to avoid
  excessive numbers of write syscalls.
diff: use std::collections::HashSet;
  -use std::io::Write;
  +use std::io::{Write, BufWriter};
  use std::fs::File;
  use std::path::{Path, PathBuf};
  @@ -53,7 +53,7 @@ pub fn output_depinfo<'a, 'b>(context: &mut Context<'a, 'b>, unit: &Unit<'a>) ->
  if let Some(link_dst) = link_dst {
  let output_path = link_dst.with_extension("d");
  let target_fn = render_filename(link_dst, basedir)?;
  - let mut outfile = File::create(output_path)?;
  + let mut outfile = BufWriter::new(File::create(output_path)?);
  write!(outfile, "{}:", target_fn)?;
  for dep in &deps {
  write!(outfile, " {}", render_filename(dep, basedir)?)?;
  