[PATCH 7 of 7 v2] histedit: automatically select root with --commands

[timeless]

> +    - Use --commands -- it will be the first linear changeset from the
> +      specified changesets.

Augie Fackler wrote:
> I'm...not sure what this means. Can you reword the help?

Imagine you have:
a->b->c->d->e->f

and you do --commands "pick f; roll d; pick c; mess e", it will use
`c` as the root.
If you did --commands "pick f; edit b; roll d; pick c; mess e", it
would use `b` as the root.

Having to manually say which the root is basically just annoying. It's
obvious as long as you aren't using automatic drops. (I really
wouldn't recommend this setting with automatic drops, although it
wouldn't be any more harmful tan not using it, in fact, it'd probably
be less harmful, since it'd only risk dropping things after you're
oldest changeset, instead of also things before it...)