`jj merge` just creates an empty change, which is practically the same
as `jj new`. The main difference is that the former requires more than
one argument and the latter requires at most one argument. It seems
cleaner to generalize them and make them aliases. This patch starts
doing that by making `jj new` accept more than one argument.
Instead of having `jj merge` be exactly an alias for `jj new`, we may
want to make it a thin wrapper that just checks that more than one
argument was given. That would probably be less confusing to users who
run `jj merge` without arguments to see what it does.
We should probably make `jj checkout` also be an alias for `jj new`,
but that will have to wait until we have removed support for open
commits (since `jj checkout` still has logic for dealing with open
commits).
