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.hs

@@ -1,10 +1,12 @@
 {- git-annex command infrastructure
  -
- - Copyright 2010-2011 Joey Hess <joey@kitenet.net>
+ - Copyright 2010-2014 Joey Hess <joey@kitenet.net>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
 
+{-# LANGUAGE BangPatterns #-}
+
 module Command (
 	command,
 	noRepo,
@@ -14,8 +16,7 @@ module Command (
 	next,
 	stop,
 	stopUnless,
-	prepCommand,
-	doCommand,
+	runCommand,
 	whenAnnexed,
 	ifAnnexed,
 	isBareRepo,
@@ -35,12 +36,13 @@ import Types.Option as ReExported
 import Seek as ReExported
 import Checks as ReExported
 import Usage as ReExported
+import RunCommand as ReExported
 import Logs.Trust
 import Config
 import Annex.CheckAttr
 
 {- Generates a normal command -}
-command :: String -> String -> [CommandSeek] -> CommandSection -> String -> Command
+command :: String -> String -> CommandSeek -> CommandSection -> String -> Command
 command = Command [] Nothing commonChecks False False
 
 {- Indicates that a command doesn't need to commit any changes to
@@ -74,25 +76,6 @@ stop = return Nothing
 stopUnless :: Annex Bool -> Annex (Maybe a) -> Annex (Maybe a)
 stopUnless c a = ifM c ( a , stop )
 
-{- Prepares to run a command via the check and seek stages, returning a
- - list of actions to perform to run the command. -}
-prepCommand :: Command -> [String] -> Annex [CommandCleanup]
-prepCommand Command { cmdseek = seek, cmdcheck = c } params = do
-	mapM_ runCheck c
-	map doCommand . concat <$> mapM (\s -> s params) seek
-
-{- Runs a command through the start, perform and cleanup stages -}
-doCommand :: CommandStart -> CommandCleanup
-doCommand = start
-  where
-	start   = stage $ maybe skip perform
-	perform = stage $ maybe failure cleanup
-	cleanup = stage $ status
-	stage = (=<<)
-	skip = return True
-	failure = showEndFail >> return False
-	status r = showEndResult r >> return r
-
 {- Modifies an action to only act on files that are already annexed,
  - and passes the key and backend on to it. -}
 whenAnnexed :: (FilePath -> (Key, Backend) -> Annex (Maybe a)) -> FilePath -> Annex (Maybe a)