[PATCH 2 of 5] commands: delay checknewlabel until after aborts

[timeless]

Yuya Nishihara wrote:
>> commands: delay checknewlabel until after aborts
> "until after aborts" ?

"abort opportunities"?

The idea was that other aborts were more meaningful.

> It doesn't make sense to me that checkformat() is still applied to known
> bookmarks. Do we want to trust known bookmarks or don't we?

with my change, checkformat is really just "strip trailing whitespace
and ensure there's something left". I've renamed it.

> This fixes https://bz.mercurial-scm.org/show_bug.cgi?id=5016 .
> Can you add a test?

It took a bit to get it to work (I shake my first at both hg export
and hg import), but yes.

> That said, I'm afraid of delaying checknewlabel() because we would have to be
> more careful to not make new labels bypass the validation.