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/Sync.hs

@@ -47,7 +47,7 @@ import Control.Concurrent.MVar
 def :: [Command]
 def = [withOptions syncOptions $
 	command "sync" (paramOptional (paramRepeating paramRemote))
-	[seek] SectionCommon "synchronize local repository with remotes"]
+	seek SectionCommon "synchronize local repository with remotes"]
 
 syncOptions :: [Option]
 syncOptions = [ contentOption ]
@@ -55,7 +55,6 @@ syncOptions = [ contentOption ]
 contentOption :: Option
 contentOption = Option.flag [] "content" "also transfer file contents"
 
--- syncing involves several operations, any of which can independently fail
 seek :: CommandSeek
 seek rs = do
 	prepMerge
@@ -78,20 +77,16 @@ seek rs = do
 	remotes <- syncRemotes rs
 	let gitremotes = filter Remote.gitSyncableRemote remotes
 
-	synccontent <- ifM (Annex.getFlag $ Option.name contentOption)
-		( withFilesInGit (whenAnnexed $ syncContent remotes) []
-		, return []
-		)
-	
-	return $ concat
-		[ [ commit ]
-		, [ withbranch mergeLocal ]
-		, map (withbranch . pullRemote) gitremotes
-		, [ mergeAnnex ]
-		, synccontent
-		, [ withbranch pushLocal ]
-		, map (withbranch . pushRemote) gitremotes
-		]
+	-- Syncing involves many actions, any of which can independently
+	-- fail, without preventing the others from running.
+	seekActions $ return [ commit ]
+	seekActions $ return [ withbranch mergeLocal ]
+	seekActions $ return $ map (withbranch . pullRemote) gitremotes
+	seekActions $ return [ mergeAnnex ]
+	whenM (Annex.getFlag $ Option.name contentOption) $
+		withFilesInGit (whenAnnexed $ syncContent remotes) []
+	seekActions $ return $ [ withbranch pushLocal ]
+	seekActions $ return $ map (withbranch . pushRemote) gitremotes
 
 {- Merging may delete the current directory, so go to the top
  - of the repo. This also means that sync always acts on all files in the