Use new append_link() API to handle long symlinks

I hit this when exporting Fedora Silverblue, there are some
long symlinks in there.

Depends: alexcrichton/tar-rs#273
Closes: ostreedev#162

Cargo.toml

@@ -7,3 +7,6 @@ opt-level = 1 # No optimizations are too slow for us.
 
 [profile.release]
 lto = "thin"
+
+[patch.crates-io]
+tar = { path = "../../alexcrichton/tar-rs" }

lib/src/tar/export.rs

@@ -231,13 +231,11 @@ impl<'a, W: std::io::Write> OstreeTarWriter<'a, W> {
                     .append_data(&mut h, &path, &mut instream)
                     .with_context(|| format!("Writing regfile {}", checksum))?;
             } else {
-                h.set_size(0);
-                h.set_entry_type(tar::EntryType::Symlink);
                 let context = || format!("Writing content symlink: {}", checksum);
-                h.set_link_name(meta.symlink_target().unwrap().as_str())
-                    .with_context(context)?;
+                h.set_entry_type(tar::EntryType::Symlink);
+                h.set_size(0);
                 self.out
-                    .append_data(&mut h, &path, &mut std::io::empty())
+                    .append_link(&mut h, &path, meta.symlink_target().unwrap().as_str())
                     .with_context(context)?;
             }
         }