GitignoreParser._iter_gitignore_files: Account for ValueError in relpath

Fixes #1317

CHANGELOG.md

@@ -5,6 +5,8 @@ Status of the `main` branch. Changes prior to the next official version change w
 * General:
   - Support environment variable `SERENA_USAGE_REPORTING` (set to `false` to disable usage reporting)
   - Extended the list of always ignored directories (by language servers) with common cases.
+  - Fix: When scanning for `.gitignore` files, the presence of files that could not be made relative 
+    to the project root would cause the scan to fail. #1317
 
 JetBrains: 
   - Improve handling of `relative_path` parameter 

src/serena/util/file_system.py

@@ -177,7 +177,11 @@ class GitignoreParser:
         while queue:
             next_abs_path = queue.pop(0)
             if next_abs_path != self.repo_root:
-                rel_path = os.path.relpath(next_abs_path, self.repo_root)
+                try:
+                    rel_path = os.path.relpath(next_abs_path, self.repo_root)
+                except ValueError:
+                    # If the path is on a different drive (Windows) or cannot be made relative for another reason, we ignore it
+                    continue
                 if self.should_ignore(rel_path):
                     continue
             yield from scan(next_abs_path)