safer inannex checking

git-annex-shell inannex now returns always 0, 1, or 100 (the last when
it's unclear if content is currently in the index due to it currently being
moved or dropped).

(Actual locking code still not yet written.)

Command/InAnnex.hs

@@ -19,8 +19,9 @@ seek :: [CommandSeek]
 seek = [withKeys start]
 
 start :: Key -> CommandStart
-start key = do
-	present <- inAnnex key
-	if present
-		then stop
-		else liftIO exitFailure
+start key = inAnnexSafe key >>= dispatch
+	where
+		dispatch (Just True) = stop
+		dispatch (Just False) = exit 1
+		dispatch Nothing = exit 100
+		exit n = liftIO $ exitWith $ ExitFailure n