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

[Glenn Hutchings <zondo42@gmail.com>]

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.

Hidden revisions are a fairly recent mercurial feature. By default, 
they're not displayed; hg needs a "--hidden" argument to consider them. 
This makes the changeset numbers non-consecutive. Here's an example:

changeset: 2:8ba76001308a
tag: tip
user: Glenn Hutchings <zondo42@gmail.com>
date: Fri Nov 27 11:11:06 2015 +0000
summary: Add a line.

changeset: 0:db414dd2e5c1
user: Glenn Hutchings <zondo42@gmail.com>
date: Fri Nov 27 11:10:31 2015 +0000
summary: Add file.

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

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

In GNU Emacs 24.3.1 (x86_64-pc-linux-gnu, GTK+ Version 3.10.7)
of 2014-03-07 on lamiak, modified by Debian
Windowing system distributor `The X.Org Foundation', version 11.0.11501000
System Description: Linux Mint 17.2 Rafaela

Configured using:
`configure '--build' 'x86_64-linux-gnu' '--build' 'x86_64-linux-gnu'
'--prefix=/usr' '--sharedstatedir=/var/lib' '--libexecdir=/usr/lib'
'--localstatedir=/var/lib' '--infodir=/usr/share/info'
'--mandir=/usr/share/man' '--with-pop=yes'
'--enable-locallisppath=/etc/emacs24:/etc/emacs:/usr/local/share/emacs/24.3/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/24.3/site-lisp:/usr/share/emacs/site-lisp' 

'--with-crt-dir=/usr/lib/x86_64-linux-gnu' '--with-x=yes'
'--with-x-toolkit=gtk3' '--with-toolkit-scroll-bars'
'build_alias=x86_64-linux-gnu' 'CFLAGS=-g -O2 -fstack-protector
--param=ssp-buffer-size=4 -Wformat -Werror=format-security -Wall'
'LDFLAGS=-Wl,-Bsymbolic-functions -Wl,-z,relro'
'CPPFLAGS=-D_FORTIFY_SOURCE=2''

Important settings:
value of $LANG: en_GB
locale-coding-system: iso-latin-1-unix
default enable-multibyte-characters: t

Major mode: Fundamental

Minor modes in effect:
tooltip-mode: t
mouse-wheel-mode: t
tool-bar-mode: t
menu-bar-mode: t
file-name-shadow-mode: t
global-font-lock-mode: t
blink-cursor-mode: t
auto-composition-mode: t
auto-encryption-mode: t
auto-compression-mode: t
buffer-read-only: t
line-number-mode: t
transient-mark-mode: t

Recent input:
<help-echo> <help-echo> <help-echo> <help-echo> <help-echo>
M-x r e p o r t - e m a c s - b u g <return>

Recent messages:
Loading 00debian-vars...done
Loading /etc/emacs/site-start.d/50a2ps.el (source)...done
Loading /etc/emacs/site-start.d/50autoconf.el (source)...done
Loading /etc/emacs/site-start.d/50cmake-data.el (source)...done
Loading /etc/emacs/site-start.d/50dictionaries-common.el (source)...
Loading /var/cache/dictionaries-common/emacsen-ispell-dicts.el (source)...
Error while loading 50dictionaries-common: Symbol's value as variable is 
void: debian-aspell-only-dictionary-alist
Loading /etc/emacs/site-start.d/50ledger.el (source)...done
For information about GNU Emacs and the GNU system, type C-h C-a.

Load-path shadows:
/usr/share/emacs24/site-lisp/cmake-data/cmake-mode hides 
/usr/share/emacs/site-lisp/cmake-mode
/usr/share/emacs/24.3/site-lisp/debian-startup hides 
/usr/share/emacs/site-lisp/debian-startup

Features:
(shadow sort gnus-util mail-extr emacsbug message format-spec rfc822 mml
easymenu mml-sec mm-decode mm-bodies mm-encode mail-parse rfc2231
mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums
mm-util mail-prsvr mail-utils time-date tooltip ediff-hook vc-hooks
lisp-float-type mwheel x-win x-dnd tool-bar dnd fontset image regexp-opt
fringe tabulated-list newcomment lisp-mode register page menu-bar
rfn-eshadow timer select scroll-bar mouse jit-lock font-lock syntax
facemenu font-core frame cham georgian utf-8-lang misc-lang vietnamese
tibetan thai tai-viet lao korean japanese hebrew greek romanian slovak
czech european ethiopic indian cyrillic chinese case-table epa-hook
jka-cmpr-hook help simple abbrev minibuffer loaddefs button faces
cus-face macroexp files text-properties overlay sha1 md5 base64 format
env code-pages mule custom widget hashtable-print-readable backquote
make-network-process dbusbind dynamic-setting system-font-setting
font-render-setting move-toolbar gtk x-toolkit x multi-tty emacs)