add "nothing" to preferred content DSL

Same as "not anything"; will be particularly useful in annex.largefiles
gitattributes.

Annex/FileMatcher.hs

@@ -74,6 +74,7 @@ parseToken matchstandard matchgroupwanted checkpresent checkpreferreddir getgrou
 	| t == "inpreferreddir" = use checkpreferreddir
 	| t == "unused" = Right $ Operation limitUnused
 	| t == "anything" = Right $ Operation limitAnything
+	| t == "nothing" = Right $ Operation limitNothing
 	| otherwise = maybe (Left $ "near " ++ show t) use $ M.lookup k $
 		M.fromList
 			[ ("include", limitInclude)

debian/changelog

@@ -5,6 +5,7 @@ git-annex (6.20160127) UNRELEASED; urgency=medium
   * Fix reversion in lookupkey, contentlocation, and examinekey which
     caused them to sometimes output side messages.
   * webapp: Fix deletion of current repository directory.
+  * Added "nothing" to preferred content expression syntax.
 
  -- Joey Hess <id@joeyh.name>  Thu, 28 Jan 2016 13:53:09 -0400
 

doc/git-annex-preferred-content.mdwn

@@ -218,6 +218,10 @@ elsewhere to allow removing it).
 
   Matches any version of any file.
 
+* `nothing`
+
+  Matches nothing. (Same as "not anything")
+
 * `not expression`
 
   Inverts what the expression matches. For example, `not include=archive/*`

doc/preferred_content.mdwn

@@ -58,6 +58,7 @@ it assumes all files that are currently present are preferred content.
 Here are recent changes to preferred content expressions, and the version
 they were added in.
 
+* "nothing" 6.201600202
 * "anything" 5.20150616
 * "standard" 5.20140314  
   (only when used in a more complicated expression; "standard" by