Fix gitignore anchored pattern logic and update test

- Use correct anchoring logic: only patterns starting with '/' are anchored
- Fix test expectation for negation patterns to match Git behavior
- Apply code formatting

src/serena/util/file_system.py

@@ -205,9 +205,6 @@ class GitignoreParser:
             if is_anchored:
                 line = line[1:]
 
-            # Remove directory marker at the end since we do not support directory-only patterns
-            line = line.removesuffix("/")
-
             # Adjust pattern based on gitignore file location
             if rel_dir:
                 if is_anchored:

test/serena/util/test_file_system.py

@@ -369,7 +369,7 @@ debug.log
 
         assert "*.log" in patterns
         assert "!important.log" in patterns
-        assert "!/src/keep.log" in patterns
+        assert "!src/keep.log" in patterns
 
     def test_comments_and_empty_lines(self):
         """Test that comments and empty lines are ignored."""