bug#22032: 24.3; VC doesn't handle hg hidden revisions

[Dmitry Gutov <dgutov@yandex.ru>]

On 11/27/2015 01:36 PM, Glenn Hutchings wrote:
> In the *vc-change-log* buffer, pressing '=' should display the diff
> twixt the current and previous revisions. This fails when (1) the HG
> backend is being used, and (2) the mercurial change history contains
> hidden revisions.

Could you link to some relevant documentation? The closest I've been 
able to find mentions "secret changesets".

> Hidden revisions are a fairly recent mercurial feature. By default,
> they're not displayed; hg needs a "--hidden" argument to consider them.

Is there a reason why we wouldn't want to use that argument and just 
always display them?

> The lisp function to find the previous rev number
> (vc-hg-previous-revision) just decrements the current rev by 1. In the
> example above, doing a diff on changeset 2 would try to use the hidden
> revision 1 and then mercurial gives an error to the effect of "rev 1 is
> hidden" (sorry, don't have the exact text to hand).

Would calling 'hg diff --hidden' help?

> Possible fix:
> actually move to the previous rev in the buffer, and get the rev number
> from that.

Unfortunately, vc-hg-previous-revision is not always called from the 
vc-log buffer. E.g. it's also called from the vc-annotate buffer. So 
we'd need a more reliable way to detect the previous revision. Such as 
an Hg command line invocation, probably.

> To repeat it: get a mercurial repo with hidden changeset (as above) and
> try the command with latest mercurial.

A publicly available example would be nice.