working_copy: print warning about ignored symlinks instead of failing build

The project doesn't currently build on Windows. One reason is because
we had a `unimplemented!()` when trying to write a symlink. Let's
print a warning instead, so the project can start building on
Windows. (The next patch will fix another build problem on Windows.)

lib/src/working_copy.rs

@@ -381,7 +381,7 @@ impl TreeState {
         create_parent_dirs(disk_path);
         #[cfg(windows)]
         {
-            unimplemented!();
+            println!("ignoring symlink at {:?}", path);
         }
         #[cfg(not(windows))]
         {