feat: add commit_date field to shuttle_index.json

The commit date provides an easy method to compare two shuttle_index.json files and detect which of the files relates to a newer commit. This enables us to prevent a situation where an earlier shuttle_index overrides a later one.

shuttle.py

@@ -79,6 +79,7 @@ class ShuttleConfig():
             "shuttle": self.config["name"],
             "repo": list(repo.remotes[0].urls)[0],
             "commit": repo.commit().hexsha,
+            "commit_date": repo.commit().committed_date,
             "mux": mux_index,
         }