add debug trace to relative_path

Summary:
i'm running into an issue with reindeer building the buck2 examples in github actions on windows. the 'buckify' command fails with
```
        thread '<unnamed>' panicked at src\buckify.rs:111:30:
        root dir not prefix of other?
```
adding this debug trace will hopefully be useful in working out what's going wrong.

Reviewed By: dtolnay

Differential Revision: D52963374

fbshipit-source-id: 3aeb88141dc2d3b15c9c20b8b54cd576e73e2ea4

src/buckify.rs

@@ -107,6 +107,12 @@ pub fn relative_path(mut base: &Path, to: &Path) -> PathBuf {
     let mut res = PathBuf::new();
 
     while !to.starts_with(base) {
+        log::debug!(
+            "relative_path: to={}, base={}, res={}",
+            to.display(),
+            base.display(),
+            res.display()
+        );
         res.push("..");
         base = base.parent().expect("root dir not prefix of other?");
     }