Make format_path behave the same for absolute paths with and without escape=True

Author: Ludvig Kihlman

nbgrader/coursedir.py

@@ -276,9 +276,12 @@ def format_path(self, nbgrader_step: str, student_id: str, assignment_id: str, e
         )
 
         if escape:
-            base = re.escape(self.root)
             structure = [x.format(**kwargs) for x in full_split(self.directory_structure)]
-            path = re.escape(os.path.sep).join([base] + structure)
+            if len(structure) == 0 or not structure[0].startswith(os.sep):
+                base = [re.escape(self.root)]
+            else:
+                base = []
+            path = re.escape(os.path.sep).join(base + structure)
         else:
             path = os.path.join(self.root, self.directory_structure.format(**kwargs))