bug#4677: allow VC operations from dired

[Dan Nicolaescu <dann@ics.uci.edu>]

Juri Linkov <juri@jurta.org> writes:

  > >   > > This patch allows VC operations to be run from dired.
  > >   > > State changing VC operations are not supported.
  > >   > > (only the various variations of log and diff)
  > >   >
  > >   > Thanks, it works right except in one case: when called outside of
  > >   > a repository controlled directory, it traverses all subdirectories
  > >   > trying to find a repository (I think traversing subdirectories
  > >   > makes no sense in this case) and later fails with:
  > >   >
  > >   >   Wrong type argument: stringp, nil
  > >
  > > Thanks.  Here's an updated patch.
  > 
  > Hmm, now it fails with "Directory not under VC" in all directories
  > even under version control since (vc-backend default-directory)
  > always returns nil.

The version that uses vc-responsible-backend should work correctly now
(i.e. it will throw an error) when used in a non VC controlled directory.