add forget command

Works, more or less. --dead is not implemented, and so far a new branch
is made, but keys no longer present anywhere are not scrubbed.

git annex sync fails to push the synced/git-annex branch after a forget,
because it's not a fast-forward of the existing synced branch. Could be
fixed by making git-annex sync use assistant-style sync branches.

Command/Forget.hs

@@ -0,0 +1,41 @@
+{- git-annex command
+ -
+ - Copyright 2013 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Command.Forget where
+
+import Common.Annex
+import Command
+import qualified Annex.Branch as Branch
+import Logs.Transitions
+import qualified Annex
+
+import Data.Time.Clock.POSIX
+
+def :: [Command]
+def = [command "forget" paramNothing seek
+		SectionMaintenance "prune git-annex branch history"]
+
+seek :: [CommandSeek]
+seek = [withNothing start]
+
+start :: CommandStart
+start = do
+	showStart "forget" "git-annex"
+	next $ perform =<< Annex.getState Annex.force
+
+perform :: Bool -> CommandPerform
+perform True = do
+	now <- liftIO getPOSIXTime
+	let ts = addTransition now ForgetGitHistory noTransitions
+	recordTransitions Branch.change ts
+	-- get branch committed before contining with the transition
+	Branch.update
+	void $ Branch.performTransitions ts True
+	next $ return True
+perform False = do
+	showLongNote "To forget git-annex branch history, you must specify --force. This deletes metadata!"
+	stop