maintain set of files pendingAdd

Kqueue needs to remember which files failed to be added due to being open,
and retry them. This commit gets the data in place for such a retry thread.

Broke KeySource out into its own file, and added Eq and Ord instances
so it can be stored in a Set.

Types/KeySource.hs

@@ -0,0 +1,33 @@
+{- KeySource data type
+ -
+ - Copyright 2012 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Types.KeySource where
+
+import Data.Ord
+
+{- When content is in the process of being added to the annex,
+ - and a Key generated from it, this data type is used. 
+ -
+ - The contentLocation may be different from the filename
+ - associated with the key. For example, the add command
+ - temporarily puts the content into a lockdown directory
+ - for checking. The migrate command uses the content
+ - of a different Key. -}
+data KeySource = KeySource
+	{ keyFilename :: FilePath
+	, contentLocation :: FilePath
+	}
+	deriving (Show)
+
+{- KeySources are assumed to be equal when the same filename is associated
+ - with the key. The contentLocation can be a random temp file.
+ -}
+instance Eq KeySource where
+	x == y = keyFilename x == keyFilename y
+
+instance Ord KeySource where
+	compare = comparing keyFilename