, but it doesn't. Instead, it ignores both "branch" and "error" and
rebases the current HEAD. This is because we try to match the number
of remainging arguments "$#", which fails to match "1" argument and
matches the "*" that really should have been a "0".
Make sure we display usage information when too many arguments are
given. Also fail-fast in case of similar bugs in the future by
matching on exactly 0 arguments and failing on unknown numbers.