fix: prevent duplicate files when --files-changed-only=false (#164)

I found that the glob pattern for some extensions hit multiple files more than once.

```py
pathlib.Path(".").rglob("*.c")
# matches demo.c and demo.cpp
```

Worse, the duplicates were being analyzed more than once.
So, this offer a performance improvement as well.

Author: 2bndy5

cpp_linter/common_fs/file_filter.py

@@ -166,7 +166,7 @@ def list_source_files(self) -> List[FileObj]:
         :returns: A list of `FileObj` objects without diff information.
         """
 
-        files = []
+        files = set()
         for ext in self.extensions:
             for rel_path in Path(".").rglob(f"*.{ext}"):
                 for parent in rel_path.parts[:-1]:
@@ -176,8 +176,8 @@ def list_source_files(self) -> List[FileObj]:
                     file_path = rel_path.as_posix()
                     logger.debug('"./%s" is a source code file', file_path)
                     if self.is_source_or_ignored(rel_path.as_posix()):
-                        files.append(FileObj(file_path))
-        return files
+                        files.add(file_path)
+        return [FileObj(f) for f in files]
 
 
 class TidyFileFilter(FileFilter):