Add a method to write a link name without canonicalization

In https://github.com/ostreedev/ostree we generate a cryptographic
checksum over files and symlinks, and directories.

ostree does not currently perform any canonicalization on symlinks;
we'll respect and honor whatever bytes we're provided as input,
and replicate that on the target.

I'm using this crate to do tar serialization, which has so far worked
fine...except, I hit this corner case:

```
[root@cosa-devsh ~]# rpm -qf /usr/lib/systemd/systemd-sysv-install
chkconfig-1.13-2.el8.x86_64
[root@cosa-devsh ~]# ll /usr/lib/systemd/systemd-sysv-install
lrwxrwxrwx. 2 root root 24 Nov 29 18:08 /usr/lib/systemd/systemd-sysv-install -> ../../..//sbin/chkconfig
[root@cosa-devsh ~]#
```

But, using `set_link_name` to write the tarball, we end up with
the canonicalized path `../../../sbin/chkconfig` - i.e. without the
double `//`.  This breaks the checksum.

Now, I am a bit tempted to change ostree to do canonicalization.  But
even if we did, I'd need to *exactly* match what this crate is doing.

In the end, what I think is useful here is a method which skips
the canonicalization and internal error checking and just writes
out into the tar stream what's requested.

(I may of course also try to change the rhel8 systemd package, but
 that's going to take a while to propagate and this corner case isn't
 the only one I'm sure)

src/header.rs

@@ -435,6 +435,18 @@ impl Header {
         })
     }
 
+    /// Sets the link name for this header without any transformation.
+    ///
+    /// This function is like [`Self::set_link_name`] but accepts an arbitrary byte array.
+    /// Hence it will not perform any canonicalization, such as replacing duplicate `//` with `/`.
+    pub fn set_link_name_literal<P: AsRef<[u8]>>(&mut self, p: P) -> io::Result<()> {
+        self._set_link_name_literal(p.as_ref())
+    }
+
+    fn _set_link_name_literal(&mut self, bytes: &[u8]) -> io::Result<()> {
+        copy_into(&mut self.as_old_mut().linkname, bytes)
+    }
+
     /// Returns the mode bits for this file
     ///
     /// May return an error if the field is corrupted.

tests/all.rs

@@ -935,6 +935,28 @@ fn long_linkname_gnu() {
     }
 }
 
+#[test]
+fn linkname_literal() {
+    for t in [tar::EntryType::Symlink, tar::EntryType::Link] {
+        let mut b = Builder::new(Vec::<u8>::new());
+        let mut h = Header::new_gnu();
+        h.set_entry_type(t);
+        h.set_size(0);
+        let path = "usr/lib/systemd/systemd-sysv-install";
+        let target = "../../..//sbin/chkconfig";
+        h.set_link_name_literal(target).unwrap();
+        t!(b.append_data(&mut h, path, std::io::empty()));
+
+        let contents = t!(b.into_inner());
+        let mut a = Archive::new(&contents[..]);
+
+        let e = &t!(t!(a.entries()).next().unwrap());
+        assert_eq!(e.header().entry_type(), t);
+        assert_eq!(e.path().unwrap().to_str().unwrap(), path);
+        assert_eq!(e.link_name().unwrap().unwrap().to_str().unwrap(), target);
+    }
+}
+
 #[test]
 fn encoded_long_name_has_trailing_nul() {
     let td = t!(TempBuilder::new().prefix("tar-rs").tempdir());