bug#13405: 24.3; log-view-diff-changeset reports wrong results

[Andy Moreton]

Emacs trunk built from r111470 on Windows XP (with mingw gcc 4.7.2).

Using vc support in emacs with a mercurial (hg) repository:
1) Annotate a file under hg version control.
2) Type "l" (vc-annotate-show-log-revision-at-line) in the annotate buffer
   to show the *vc-change-log* buffer containing the log entry for
   revision 46769.
3) In the *vc-change-log* buffer, type D (log-view-diff-changeset) to view
   the changeset for that single revision.
   Emacs runs "hg diff --git -r 46768 -r 46769", but this does not show
   the expected contents of the changeset, as the previous revision was
   on a different named branch. Instead this shows the (large) diff between
   the two branches. The line counts illustrate this:

   # hg diff -r 46768 -r 46769 | wc -l   ## Diff between branches
   102276
   # hg diff -c 46769 | wc -l            ## Changeset diff
   295
   # hg log -p -r 46769 | wc -l          ## Changeset diff + hg metadata
   302

   The second and third commands give the expected output.
   Can log-view-diff-changeset be changed to deal with this case more
   gracefully ?

       AndyM