add remote fsck interface

Currently only implemented for local git remotes. May try to add support
to git-annex-shell for ssh remotes later. Could concevably also be
supported by some special remote, although that seems unlikely.

Cronner user this when available, and when not falls back to
fsck --fast --from remote

git annex fsck --from does not itself use this interface.
To do so, I would need to pass --fast and all other options that influence
fsck on to the git annex fsck that it runs inside the remote. And that
seems like a lot of work for a result that would be no better than
cd remote; git annex fsck
This may need to be revisited if git-annex-shell gets support, since it
may be the case that the user cannot ssh to the server to run git-annex
fsck there, but can run git-annex-shell there.

This commit was sponsored by Damien Diederen.

Remote/Git.hs

@@ -42,10 +42,12 @@ import Utility.Metered
 #ifndef mingw32_HOST_OS
 import Utility.CopyFile
 #endif
+import Utility.Batch
 import Remote.Helper.Git
 import Remote.Helper.Messages
 import qualified Remote.Helper.Ssh as Ssh
 import qualified Remote.GCrypt
+import Config.Files
 
 import Control.Concurrent
 import Control.Concurrent.MSampleVar
@@ -111,6 +113,9 @@ gen r u c gc
 			, hasKey = inAnnex r
 			, hasKeyCheap = repoCheap r
 			, whereisKey = Nothing
+			, remoteFsck = if Git.repoIsUrl r
+				then Nothing
+				else Just $ fsckOnRemote r
 			, config = M.empty
 			, localpath = localpathCalc r
 			, repo = r
@@ -396,6 +401,17 @@ copyToRemote r key file p
 							(\d -> rsyncOrCopyFile params object d p)
 			)
 
+fsckOnRemote :: Git.Repo -> [CommandParam] -> Annex (IO Bool)
+fsckOnRemote r params
+	| Git.repoIsUrl r = return $ do
+		program <- readProgramFile
+		batchCommand program $ Param "fsck" : params
+	| otherwise = do
+		s <- Ssh.git_annex_shell r "fsck" params []
+		return $ case s of
+			Nothing -> return False
+			Just (c, ps) -> batchCommand c ps
+
 {- Runs an action on a local repository inexpensively, by making an annex
  - monad using that repository. -}
 onLocal :: Git.Repo -> Annex a -> IO a