string_list_append(&subjects,
                                           oid_to_hex(&commit->object.oid));
                else
-                       string_list_append(&subjects, strbuf_detach(&sb, NULL));
+                       string_list_append_nodup(&subjects,
+                                                strbuf_detach(&sb, NULL));
        }
 
        if (opts->credit_people)
        struct strbuf tagbuf = STRBUF_INIT;
 
        for (i = 0; i < origins.nr; i++) {
-               unsigned char *sha1 = origins.items[i].util;
+               struct object_id *oid = origins.items[i].util;
                enum object_type type;
                unsigned long size, len;
-               char *buf = read_sha1_file(sha1, &type, &size);
+               char *buf = read_object_file(oid, &type, &size);
                struct strbuf sig = STRBUF_INIT;
 
                if (!buf || type != OBJ_TAG)
        head_commit = lookup_commit(head);
        if (head_commit)
                commit_list_insert(head_commit, &parents);
-       parents = reduce_heads(parents);
+       reduce_heads_replace(&parents);
 
        while (parents) {
                struct commit *cmit = pop_commit(&parents);