Allow long link names in tar files

Without this, users trying to run `x.py dist` under a sufficiently long
path run into problems when we build the resulting tarballs due to
length limits in the original tar spec. The error looks like:

        Finished release [optimized + debuginfo] target(s) in 0.34s
    Copying stage0 std from stage0 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / x86_64-unknown-linux-musl)
    Building stage0 tool rust-installer (x86_64-unknown-linux-gnu)
        Finished release [optimized] target(s) in 0.35s
    Dist rust-std-1.67.1-x86_64-unknown-linux-musl
    Error: failed to generate installer

    Caused by:
        0: failed to tar file '/home/AAAAAAAAAAAAAA/BBBBBB/CCCC/DDD/EEEEE/FFFFFFFFFFFF/GGGGGGGGGGGGGGGG/HHHHHHHHHH/IIIIIIIIIIIIIII/JJJJJ/KKKKKKK/src/build/tmp/tarball/rust-std/x86_64-unknown-linux-musl/rust-std-1.67.1-x86_64-unknown-linux-musl/rust-std-x86_64-unknown-linux-musl/lib/rustlib/x86_64-unknown-linux-musl/lib/self-contained/libc.a'
        1: provided value is too long when setting link name for
    Build completed unsuccessfully in 0:00:03

The fix is to make use of the widely-supported GNU tar extensions which
lift this restriction. Switching to [`tar::Builder::append_link`] takes
care of that for us. See also alexcrichton/tar-rs#273.

[`tar::Builder::append_link`]: https://docs.rs/tar/0.4.38/tar/struct.Builder.html#method.append_link

Author: Jon Gjengset

src/tools/rust-installer/Cargo.toml

@@ -13,7 +13,7 @@ path = "src/main.rs"
 anyhow = "1.0.19"
 flate2 = "1.0.1"
 rayon = "1.0"
-tar = "0.4.13"
+tar = "0.4.38"
 walkdir = "2"
 xz2 = "0.1.4"
 num_cpus = "1"

src/tools/rust-installer/src/tarballer.rs

@@ -1,6 +1,6 @@
 use anyhow::{bail, Context, Result};
 use std::fs::{read_link, symlink_metadata};
-use std::io::{empty, BufWriter, Write};
+use std::io::{BufWriter, Write};
 use std::path::Path;
 use tar::{Builder, Header};
 use walkdir::WalkDir;
@@ -93,8 +93,7 @@ fn append_path<W: Write>(builder: &mut Builder<W>, src: &Path, path: &String) ->
     header.set_metadata(&stat);
     if stat.file_type().is_symlink() {
         let link = read_link(src)?;
-        header.set_link_name(&link)?;
-        builder.append_data(&mut header, path, &mut empty())?;
+        builder.append_link(&mut header, path, &link)?;
     } else {
         if cfg!(windows) {
             // Windows doesn't really have a mode, so `tar` never marks files executable.