add AssumeNotPresent parameter to limits

Solves the issue with preferred content expressions and dropping that
I mentioned yesterday. My solution was to add a parameter to specify a set
of repositories where content should be assumed not to be present. When
deciding whether to drop, it can put the current repository in, and then
if the expression fails to match, the content can be dropped.

Using yesterday's example "(not copies=trusted:2) and (not in=usbdrive)",
when the local repo is one of the 2 trusted copies, the drop check will
see only 1 trusted copy, so the expression matches, and so the content will
not be dropped.

Logs/PreferredContent.hs

@@ -20,7 +20,7 @@ import Common.Annex
 import qualified Annex.Branch
 import qualified Annex
 import Logs.UUIDBased
-import Limit (limitInclude, limitExclude, limitIn, limitCopies, limitInBackend)
+import Limit (MatchFiles, limitInclude, limitExclude, limitIn, limitCopies, limitInBackend)
 import qualified Utility.Matcher
 
 {- Filename of preferred-content.log. -}
@@ -56,7 +56,7 @@ preferredContentMapRaw = simpleMap . parseLog Just
  - because the configuration is shared amoung repositories and newer
  - versions of git-annex may add new features. Instead, parse errors
  - result in a Matcher that will always succeed. -}
-makeMatcher :: String -> Utility.Matcher.Matcher (FilePath -> Annex Bool)
+makeMatcher :: String -> Utility.Matcher.Matcher MatchFiles
 makeMatcher s
 	| null (lefts tokens) =  Utility.Matcher.generate $ rights tokens
  	| otherwise = Utility.Matcher.generate []
@@ -69,7 +69,7 @@ checkPreferredContentExpression s = case lefts $ map parseToken $ tokenizeMatche
 	[] -> Nothing
 	l -> Just $ unwords $ map ("Parse failure: " ++) l
 
-parseToken :: String -> Either String (Utility.Matcher.Token (FilePath -> Annex Bool))
+parseToken :: String -> Either String (Utility.Matcher.Token MatchFiles)
 parseToken t
 	| any (== t) Utility.Matcher.tokens = Right $ Utility.Matcher.token t
 	| otherwise = maybe (Left $ "near " ++ show t) use $ M.lookup k m