[MERGE] add --detritus option to ls

[John Yates]

Martin Pool writes:

> Backups are a more interesting case... by naming them foo~ we comply  
> with general practice but this does mean that deleting them will also  
> clobber any backups created by vim, emacs, etc.

Robert Collins replies:

> I consider that a feature :).

I long ago configured emacs to place my ~ backup files in a separate
directory.  Thus a claim that dropping ~ files into my workspace is a
common -- and hence presumably acceptable -- practice carries little
weight.  Could bzr provide a means of segregating such backup files in
some less obtrusive location?

/john