forget --drop-dead: Completely removes mentions of repositories that have been marked as dead from the git-annex branch.

Wrote nice pure transition calculator, and ugly code to stage its results
into the git-annex branch. Also had to split up several Log modules
that Annex.Branch needed to use, but that themselves used Annex.Branch.

The transition calculator is limited to looking at and changing one file at
a time. While this made the implementation relatively easy, it precludes
transitions that do stuff like deleting old url log files for keys that are
being removed because they are no longer present anywhere.

Command/Forget.hs

@@ -12,30 +12,41 @@ import Command
 import qualified Annex.Branch as Branch
 import Logs.Transitions
 import qualified Annex
+import qualified Option
 
 import Data.Time.Clock.POSIX
 
 def :: [Command]
-def = [command "forget" paramNothing seek
+def = [withOptions forgetOptions $ command "forget" paramNothing seek
 		SectionMaintenance "prune git-annex branch history"]
 
+forgetOptions :: [Option]
+forgetOptions = [dropDeadOption]
+
+dropDeadOption :: Option
+dropDeadOption = Option.flag [] "drop-dead" "drop references to dead repositories"
+
 seek :: [CommandSeek]
-seek = [withNothing start]
+seek = [withFlag dropDeadOption $ \dropdead ->
+	withNothing $ start dropdead]
 
-start :: CommandStart
-start = do
+start :: Bool -> CommandStart
+start dropdead = 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
+	let basets = addTransition now ForgetGitHistory noTransitions
+	let ts = if dropdead
+		then addTransition now ForgetDeadRemotes basets
+		else basets
+	next $ perform ts =<< Annex.getState Annex.force
+
+perform :: Transitions -> Bool -> CommandPerform
+perform ts True = do
 	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
+perform _ False = do
 	showLongNote "To forget git-annex branch history, you must specify --force. This deletes metadata!"
 	stop