Toon Claes [off-list ref] writes:

+static void put_mapped_commit(kh_oid_map_t *replayed_commits,
+			      struct commit *commit,
+			      struct commit *new_commit)
+{
+	khint_t pos;
+	int ret;
+
+	pos = kh_put_oid_map(replayed_commits, commit->object.oid, &ret);
+	if (ret == 0)
+		BUG("Duplicate rewritten commit: %s\n",

Please do not add terminating LF at the end of single-liner messages
that use our print infrastructure, like BUG(), warning(), error(),
and die(), as the machinery adds one for you.

Other than that, looking good.