rework git check-attr interface

Now gitattributes are looked up, efficiently, in only the places that
really need them, using the same approach used for cat-file.

The old CheckAttr code seemed very fragile, in the way it streamed files
through git check-attr.
I actually found that cad8824852
was still deadlocking with ghc 7.4, at the end of adding a lot of files.
This should fix that problem, and avoid future ones.

The best part is that this removes withAttrFilesInGit and withNumCopies,
which were complicated Seek methods, as well as simplfying the types
for several other Seek methods that had a Backend tupled in.

Command/Add.hs

@@ -16,7 +16,6 @@ import qualified Backend
 import Logs.Location
 import Annex.Content
 import Utility.Touch
-import Backend
 
 def :: [Command]
 def = [command "add" paramPaths seek "add files to annex"]
@@ -28,8 +27,8 @@ seek = [withFilesNotInGit start, withFilesUnlocked start]
 {- The add subcommand annexes a file, storing it in a backend, and then
  - moving it into the annex directory and setting up the symlink pointing
  - to its content. -}
-start :: BackendFile -> CommandStart
-start p@(_, file) = notBareRepo $ ifAnnexed file fixup add
+start :: FilePath -> CommandStart
+start file = notBareRepo $ ifAnnexed file fixup add
 	where
 		add = do
 			s <- liftIO $ getSymbolicLinkStatus file
@@ -37,7 +36,7 @@ start p@(_, file) = notBareRepo $ ifAnnexed file fixup add
 				then stop
 				else do
 					showStart "add" file
-					next $ perform p
+					next $ perform file
 		fixup (key, _) = do
 			-- fixup from an interrupted add; the symlink
 			-- is present but not yet added to git
@@ -45,8 +44,10 @@ start p@(_, file) = notBareRepo $ ifAnnexed file fixup add
 			liftIO $ removeFile file
 			next $ next $ cleanup file key =<< inAnnex key
 
-perform :: BackendFile -> CommandPerform
-perform (backend, file) = Backend.genKey file backend >>= go
+perform :: FilePath -> CommandPerform
+perform file = do
+	backend <- Backend.chooseBackend file
+	Backend.genKey file backend >>= go
 	where
 		go Nothing = stop
 		go (Just (key, _)) = do