bug#7350: 24.0.50; make vc-deduce-backend smarter

[Bob Rogers <rogers@rgrjr.dyndns.org>]

   From: Stefan Monnier <monnier@IRO.UMontreal.CA>
   Date: Wed, 17 Nov 2010 08:31:26 -0500

   > Grepping found only one caller; vc-log-edit passes it as the
   > log-edit-diff-function.  But this really ought to use the passed
   > fileset, and not go through the command again, true?

   Actually, what it *should* do is select the same fileset as the one
   that C-c C-c will commit.

Yes, exactly.

   I'm not sure which fileset is chosen by C-c C-c, but if I were to
   choose, I'd follow the PCL-CVS behavior which is to recompute the
   fileset rather than use whatever was the fileset when the *VC-Log*
   buffer was created . . .

The fileset is not recomputed; if you change the fileset in *vc-dir*,
you must do vc-next-action explicitly to update the log buffer fileset.
That threw me for a loop when I first used it, but now that I've gotten
used to it, I can see the utility of it.

   (but aditionally check whether the new fileset is the same as the old
   one, and if not prompt the user to make sure that's really what she
   intended).

	   Stefan

Sounds like a clear win, since it removes the confusingness while
continuing to allow both behaviors.  Accordingly, that ought to get
fixed first.  If I come up with a reasonable patch, I'll file a new bug
report.

					-- Bob