treat "not present" in preferred content as invalid

Detect when a preferred content expression contains "not present", which
would lead to repeatedly getting and then dropping files, and make it never
match. This also applies to "not balanced" and "not sizebalanced".

--explain will tell the user when this happens

Note that getMatcher calls matchMrun' and does not check for unstable
negated limits. While there is no --present anyway, if there was,
it would not make sense for --not --present to complain about
instability and fail to match.

Types/FileMatcher.hs

@@ -98,7 +98,9 @@ data MatchFiles a = MatchFiles
 	-- ^ does the matchAction look at the location log?
 	, matchNeedsLiveRepoSize :: Bool
 	-- ^ does the matchAction need live repo size information?
-	, matchDesc :: Bool -> MatchDesc
+	, matchNegationUnstable :: Bool
+	-- ^ does negating the matchAction lead to unstable behavior?
+	, matchDesc :: Maybe Bool -> MatchDesc
 	-- ^ displayed to the user to describe whether it matched or not
 	}