fix inversion of control in CommandSeek (no behavior changes)

I've been disliking how the command seek actions were written for some
time, with their inversion of control and ugly workarounds.

The last straw to fix it was sync --content, which didn't fit the
Annex [CommandStart] interface well at all. I have not yet made it take
advantage of the changed interface though.

The crucial change, and probably why I didn't do it this way from the
beginning, is to make each CommandStart action be run with exceptions
caught, and if it fails, increment a failure counter in annex state.
So I finally remove the very first code I wrote for git-annex, which
was before I had exception handling in the Annex monad, and so ran outside
that monad, passing state explicitly as it ran each CommandStart action.

This was a real slog from 1 to 5 am.

Test suite passes.

Memory usage is lower than before, sometimes by a couple of megabytes, and
remains constant, even when running in a large repo, and even when
repeatedly failing and incrementing the error counter. So no accidental
laziness space leaks.

Wall clock speed is identical, even in large repos.

This commit was sponsored by an anonymous bitcoiner.

Command/Unused.hs

@@ -45,8 +45,8 @@ def = [withOptions [fromOption] $ command "unused" paramNothing seek
 fromOption :: Option
 fromOption = Option.field ['f'] "from" paramRemote "remote to check for unused content"
 
-seek :: [CommandSeek]
-seek = [withNothing start]
+seek :: CommandSeek
+seek = withNothing start
 
 {- Finds unused content in the annex. -} 
 start :: CommandStart
@@ -326,14 +326,14 @@ data UnusedMaps = UnusedMaps
 	, unusedTmpMap :: UnusedMap
 	}
 
-{- Read unused logs once, and pass the maps to each start action. -}
 withUnusedMaps :: (UnusedMaps -> Int -> CommandStart) -> CommandSeek
 withUnusedMaps a params = do
 	unused <- readUnusedLog ""
 	unusedbad <- readUnusedLog "bad"
 	unusedtmp <- readUnusedLog "tmp"
 	let m = unused `M.union` unusedbad `M.union` unusedtmp
-	return $ map (a $ UnusedMaps unused unusedbad unusedtmp) $
+	let unusedmaps = UnusedMaps unused unusedbad unusedtmp
+	seekActions $ return $ map (a unusedmaps) $
 		concatMap (unusedSpec m) params
 
 unusedSpec :: UnusedMap -> String -> [Int]
@@ -349,8 +349,8 @@ unusedSpec m spec
 		_ -> badspec
 	badspec = error $ "Expected number or range, not \"" ++ spec ++ "\""
 
-{- Start action for unused content. Finds the number in the maps, and
- - calls either of 3 actions, depending on the type of unused file. -}
+{- Seek action for unused content. Finds the number in the maps, and
+ - calls one of 3 actions, depending on the type of unused file. -}
 startUnused :: String
 	-> (Key -> CommandPerform)
 	-> (Key -> CommandPerform)