Only list new files in changelog tool

This updates the changelog automation tool to only check newly-added
files in the diff for changelog entries. This prevents
false-positives when changelog entries are removed from `next-release`

Author: JordonPhillips

.changes/smithy_changelog/amend.py

@@ -142,7 +142,7 @@ def _get_new_changes(repository_dir: Path, base: str | None) -> dict[Path, Chang
     with chdir(repository_dir):
         print(f"Running a diff against base branch: {base}")
         result = subprocess.run(
-            f"git diff origin/{base} --name-only",
+            f"git diff origin/{base} --name-only --diff-filter=A",
             check=True,
             shell=True,
             capture_output=True,