repo: Fix backwards timestamp in ostree_repo_write_commit()

ostree_repo_write_commit_with_time() converts the timestamp to
big-endian byte order.

ostree_repo_write_commit() was also doing this when calling
ostree_repo_write_commit_with_time(), resulting in a corrupted
commit object (timestamp bytes were backwards).

Recent regression in 14ffd7022a

src/libostree/ostree-repo-commit.c

@@ -1978,7 +1978,7 @@ ostree_repo_write_commit (OstreeRepo      *self,
                                           body,
                                           metadata,
                                           root,
-                                          GUINT64_TO_BE (g_date_time_to_unix (now)),
+                                          g_date_time_to_unix (now),
                                           out_commit,
                                           cancellable,
                                           error);