bug#36534: 25.2; mercurial vc-annotate sometimes fails when using the evolve mercurial extension

[Valentin Gatien-Baron <valentin.gatienbaron@gmail.com>]

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Sometimes, vc-annotate in a file tracked by mercurial returns an
error like this instead of a proper annotate buffer:

abort: mercurial/merge.py: no such file in rev 9117c6561b0b

Here is a reproduction (requires the evolve extension to
be enabled, for instance as described here:
https://www.mercurial-scm.org/wiki/EvolveExtension):

  rm -rf /tmp/test
  hg init /tmp/test
  cd /tmp/test
  echo b > b
  hg commit -Am0
  echo a > a
  hg commit -Am1
  hg commit --amend -m 2
  if [ $(hg log -r . -T {rev}) = 1 ]; then
      echo "Error: cannot test as evolve is not enabled"
      exit 1
  fi
  emacs -Q a -f 'vc-annotate'


What happens is:
1. vc-annotate calls vc-working-revision
2. vc-hg-working-revision calls hg parent --template {rev} file
3. that command fails saying something like "abort: filtered revision
'3ee3303ecf2621381e5d5ca14aba73f2835facb5'!"
4. vc-hg-working-revision thus returns 0
5. vc-annotate calls `hg annotate -r 0 file`, which correctly errors
out, saying "abort: file: no such file in hex-rev-of-rev-0".

Step 3 is probably an hg bug. However `hg parent` was deprecated in
2014, so people may be reluctant to change it. And it would be simpler
for vc-hg-working-revision to call `hg log -r . -T {rev}` (vc-git does
that, so I assume that it works in general on top of fixing the
current issue). `.` means the current revision, see `hg help revset`.

So something like this:
(defun vc-hg-working-revision (file)
  "Hg-specific version of `vc-working-revision'."
  (ignore-errors
    (with-output-to-string
      (vc-hg-command standard-output 0 nil
    "log" "-r" "." "--template" "{rev}"))))

(another reason to stop using `hg parents` is that during a merge, it
will in most cases print two revisions, so if you annotate a file that
was last changed in revision 123, emacs will likely get 123123 back
and `hg annotate -r 123123` will fail)



In GNU Emacs 25.2.2 (x86_64-pc-linux-gnu, GTK+ Version 3.23.2)
 of 2018-08-27, modified by Debian built on lgw01-amd64-037
Windowing system distributor 'The X.Org Foundation', version 11.0.12001000
System Description: Ubuntu 18.10

Configured using:
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Configured features:
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Important settings:
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Major mode: Python

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Recent messages:
Loading debian-ispell...done
Loading /var/cache/dictionaries-common/emacsen-ispell-dicts.el
(source)...done
Loading /etc/emacs/site-start.d/50dictionaries-common.el (source)...done
Loading /etc/emacs/site-start.d/50ocaml-mode.el (source)...done
Loading /etc/emacs/site-start.d/51tuareg-mode.el (source)...done
For information about GNU Emacs and the GNU system, type C-h C-a.
Annotating...
vc-do-command: Running hg annotate -dq -n -u --follow -r0 merge.py...FAILED
(status 255)
Mark set
Making completion list...

Load-path shadows:
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Memory information:
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