bug#2872: diff from log-view not working for bzr

[Dan Nicolaescu]

Do a C-x v l on any file under bzr.

Put the point on some random log entry and type 
d
And empty *vc-diff* window will appear.

The problem is that vc-bzr-diff gets called with the arguments REV1 == REV2.
And that happens because of this code in log-view-diff:

        (log-view-msg-next)
        (setq to (log-view-current-tag))  [1]

Given a log entry like this:

------------------------------------------------------------
revno: 295


`log-view-msg-next' puts the point at the beginning of the ----
line and `log-view-current-tag' returns the version of the _previous_
log entry.  It happens because `log-view-current-tag' first does a 
(forward-line 1) --> point will be at "revno" after that.
And then (re-search-backward log-view-message-re nil t)
which will find the current log entry.

So in conclusion code [1] fails to advance and find the next log
entry.