+static int try_parent_shorthands(const char *arg)
+{
+ char *dotdot;
+ unsigned char sha1[20];
+ struct commit *commit;
+ struct commit_list *parents;
+ int parents_only;
+
+ if ((dotdot = strstr(arg, "^!")))
+ parents_only = 0;
+ else if ((dotdot = strstr(arg, "^@")))
+ parents_only = 1;
+
+ if (!dotdot || dotdot[2])
+ return 0;
+
+ *dotdot = 0;
+ if (get_sha1(arg, sha1))
+ return 0;
+
+ if (!parents_only)
+ show_rev(NORMAL, sha1, arg);
+ commit = lookup_commit_reference(sha1);
+ for (parents = commit->parents; parents; parents = parents->next)
+ show_rev(parents_only ? NORMAL : REVERSED,
+ parents->item->object.sha1, arg);
+
+ return 1;
+}
+