bug#63949: 30.0.50; `vc-print-log´ does not erase buffer when called from *vc-change-log* buffer, at least for CVS logs

[Jens Schmidt via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Start emacs -Q.  Visit a CVS-controlled file.  Then:

C-x v l
M->
C-x v l

Notice the CVS log added once more after current end of log buffer.

I tracked this down to the following snippet from `vc-do-command´:

     (save-current-buffer
       (unless (or (eq buffer t)
		  (and (stringp buffer)
		       (string= (buffer-name) buffer))
		  (eq buffer (current-buffer)))
         (vc-setup-buffer buffer))

That is, `vc-setup-buffer´ (which erases the buffer) is *not* called if 
the buffer where the VC command is to be executed in equals the current 
buffer.  I guess that logic has a reason, so it probably should be some 
upper layer which should erase *vc-log-buffer*...

For git, for example, function `vc-git-print-log´ calls 
`vc-setup-buffer´.  Probably the other VC backends should follow suit?

If this is the way to go, I could provide patches for VC backends CVS 
and RCS, pls let me know.



In GNU Emacs 30.0.50 (build 2, x86_64-pc-linux-gnu, GTK+ Version
  3.24.24, cairo version 1.16.0) of 2023-06-07 built on sappc2
Repository revision: a902156068ab071f93cc9bbd34cd320919b74064
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Debian GNU/Linux 11 (bullseye)

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