bug#13004: 24.2; vc-diff from vc-log doesn't show diff before renames (for Mercurial backend).

[Oleksandr Gavenko <gavenkoa@gmail.com>]

Additional info.

As we get know that HG doesn't allow follow history for HG another Emacs vc
module work as not expected.

So make test repo:

  mkdir /tmp/test
  cd /tmp/test
  hg init
  echo xxx >>README
  hg add
  hg ci -m init
  echo yyy >>README
  hg ci -m before
  hg mv README INSTALL
  hg ci -m rename
  echo zzz >>INSTALL
  hg ci -m after

open INSTALL file and annotate content (C-x v g runs the command vc-annotate).
Next go to previous version by "a" (vc-annotate-revision-previous-to-line) on
second line:

  0 Tue Nov 27 21:12:00 2012 +0200  README: xxx
  1 Tue Nov 27 21:12:23 2012 +0200  README: yyy          (*1*)
  3 Tue Nov 27 21:13:15 2012 +0200 INSTALL: zzz

So you move to:

  0 Tue Nov 27 21:12:00 2012 +0200 README: xxx           (*2*)

If you try move forward by "w" (vc-annotate-working-revision) Emacs print
error:

  vc-annotate-warp-revision: Invalid argument to vc-annotate-warp-revision

While in case of SVN we back to original state (*1*).

-- 
Best regards!