bug#5961: 23.1 regression: vc-hg does not show 0 version number for 'added files

[Chong Yidong]

Dan Nicolaescu <dann@gnu.org> writes:

> After this change:
>
>   Show working revision correctly for mercurial.
>   * vc-hg.el (vc-hg-working-revision): Use hg parent instead of
>   hg log as suggested by Alex Harsanyi <alexharsanyi@gmail.com>,
>
> the mode-line show Hg@ instead of Hg@0 for files in the VC 'added
> state.  This is a regression from 23.1.
>
> The problem is that the "hg parent " command does not distinguish
> between the 'added and 'unregistered files.
>
> Should the patch below go into the 23.2 branch?

Yes please.  I don't have hg installed at the moment, so it's
inconvenient for me to double check your work, but if you're confident
it's correct then please go ahead and commit it.

Thanks.