bug#37223: 26.2; vc does not honor backup-directory-alist

[ndame]

> 
> >
> > These are backup files, the vc function uses that name too
> > (vc-version-backup-file-name), so their placements should honor
> > backup-directory-alist which is currently ignored.
> 
> I don't know if they should. The current behavior is a long-standing
> one.  But more opinions are welcome.

The main problem is the user may not want these files at all.

I usually look at a particular version then close the buffer, I don't
want a file created and having such files polluting the directory. I
just want to see the contents.

I'd say more people use log to look up something in a certain version
than actually wanting to get a certain version on disk.

So there's two possible solutions:

1. Add an option to control if backup-directory-alist is honored by
these files and this can be off by default to keep current default
behavior.

2. Do not create files at all on disk, just read the selected
version's contents into a new buffer. The user can decide if he wants
to just look at the file and then close the buffer, or if he wants the
given version on disk then he can save the buffer.

The 2nd option seems better, since it gives control to the user to do
with the buffer as he wishes.