Assure cross-fs check doesn't break if cursor is currently empty

A pop on the cursor might leave it empty, which usually isn't a problem
as we push paths on top of it for file tests. Trying to get metadata
on an empty path is invalid though, so we replace it with '.' if needed.

Author: Byron

git-discover/src/upwards.rs

@@ -133,7 +133,13 @@ pub(crate) mod function {
 
             #[cfg(unix)]
             if current_height != 0 && !cross_fs {
-                let metadata = cursor.metadata().map_err(|_| Error::InaccessibleDirectory {
+                let metadata = if cursor.as_os_str().is_empty() {
+                    Path::new(".")
+                } else {
+                    cursor.as_ref()
+                }
+                .metadata()
+                .map_err(|_| Error::InaccessibleDirectory {
                     path: cursor.to_path_buf(),
                 })?;