Add plumbing-level lookupkey command.

Command/LookupKey.hs

@@ -0,0 +1,26 @@
+{- git-annex command
+ -
+ - Copyright 2013 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Command.LookupKey where
+
+import Common.Annex
+import Command
+import Annex.CatFile
+import Types.Key
+
+def :: [Command]
+def = [notBareRepo $
+	command "lookupkey" (paramRepeating paramFile) seek
+		SectionPlumbing "looks up key used for file"]
+
+seek :: [CommandSeek]
+seek = [withStrings start]
+
+start :: String -> CommandStart
+start file = do
+	liftIO . maybe exitFailure (putStrLn . key2file) =<< catKeyFile file
+	stop