bug#14989: 24.3.50; log-view-diff and log-view-diff-changeset default to different `to' revisions for git merge commits

[Dmitry Gutov <dgutov@yandex.ru>]

To reproduce:

Have a merge commit where the second branch (merged from) has a newer
last commit than the first branch (merged to).

`vc-print-root-log' buffer would look like this:

*  (HEAD, devel)b044fba..: Some Guy 2013-07-25 foo foo
*   2152d76..: Dmitry Gutov 2013-07-25 Merge branch 'bar' into devel
|\
| *  (bar)1554943..: Another Guy 2013-07-25 Abc
| * f92770a..: Another Guy 2013-07-25 Def
* | 5f5c068..: Another Guy 2013-07-25 Ghi

If I move to commit 2152d76 and press `d', it will show the diff against
1554943. If I press `D', if will show the diff against 5f5c068, which is
*the* parent commit, as far as Git is concerned (the first parent, to be
more exact), because it belongs to the branch `devel', and the merge was
done from `bar' to `devel'.

Since this is a `vc-print-root-log' buffer, both commands will show the
full changesets, not just changes in an individual file.

But the behavior in `vc-print-log' is the same in this respect, except
the diff `d' shows is just for the current file.

`log-view-diff' behavior makes certain amount of sense (it always diffs
against the revision on the line below), but it's inconsistent with
`log-view-diff-changeset', and it also backfires when the current commit
is not a child of the commit on the previous line. Example: f92770a and
5f5c068. As a result, we get a meaningless diff.

This fix is simple enough. Apply?

=== modified file 'lisp/vc/log-view.el'
--- lisp/vc/log-view.el 2013-06-11 06:36:06 +0000
+++ lisp/vc/log-view.el 2013-07-30 22:31:40 +0000
@@ -565,10 +565,7 @@
   (let ((fr (log-view-current-tag beg))
         (to (log-view-current-tag end)))
     (when (string-equal fr to)
-      (save-excursion
-        (goto-char end)
-        (log-view-msg-next)
-        (setq to (log-view-current-tag))))
+      (setq to (vc-call-backend log-view-vc-backend 'previous-revision nil fr)))
     (vc-diff-internal
      t (list log-view-vc-backend
             (if log-view-per-file-logs

The functions could also use some further cleanup:
1) Swap the `to' and `fr' local variables.
2) Extract the function bodies to one function with a third argument
(called `whole-changeset', maybe), and make them call it.

Objections?