--anything and --nothing

--anything and --nothing

Added --anything (and --nothing). Eg, git-annex find --anything will list
all annexed files whether or not the content is present. This is slightly
faster and clearer than --include=* or --exclude=*

While I can't imagine how --nothing will be used, preferred content
expressions already had anything and nothing, so might as well support both
as matching options as well.

Sponsored-by: Dartmouth College's Datalad project