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

[Stefan Monnier]

>    It can definitely be generalized, but I'd rather stick to buffers where
>    there's a clear association with a particular file or directory.
>    E.g. *Help* buffers aren't good candidates.
> Fair enough.  Seems odd that default-directory is not nil for these.

There are all kinds of reasons why we don't like default-directory to be nil.

> 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.

>    This was the result of a trade-off (get rid of one RET since it's
>    almost never used).  But I guess we could/should prompt the user for
>    a file/dir rather than signal "File is not under version control" or
>    some such error.

> That would be much nicer, especially since the underlying functionality
> (i.e. support for vc-diff of directories) still works.

Patch welcome.

> P.S.  I mailed the signed copyright assignment today, so FSF should have
> it tomorrow.

Great (tho, OT1H I can't remember for which patch this is, and OTOH the
FSF itself usually takes a little while to process those mail).


        Stefan