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

[Bob Rogers]

   From: Stefan Monnier <monnier@iro.umontreal.ca>
   Date: Fri, 12 Nov 2010 08:48:30 -0500

   > Works for me.  It doesn't cover the cases where I'm looking at a
   > generated file, or test output, but (in my workflow anyway) those cases
   > are rarer.

   I've installed the patch into the trunk (i.e. not for Emacs-23.3 but
   Emacs-24) and added compilation-mode to shell-mode, since it's also
   clearly linked to a particular location in the file-system.

FWIW, the "32.1.4 Examining And Comparing Old Revisions" section of the
Emacs info documentation says the following (tenth paragraph):

       If you invoke `C-x v =' or `C-u C-x v =' from a buffer that is
    neither visiting a version-controlled file nor a VC directory buffer,
    these commands generate a diff of all registered files in the current
    directory and its subdirectories.

So it turns out that my expectation is what is actually documented after
all.  ;-}

                                        -- Bob