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.hs

@@ -18,6 +18,7 @@ module Command (
 	ifAnnexed,
 	notBareRepo,
 	isBareRepo,
+	numCopies,
 	autoCopies,
 	module ReExported
 ) where
@@ -34,6 +35,7 @@ import Checks as ReExported
 import Usage as ReExported
 import Logs.Trust
 import Config
+import Annex.CheckAttr
 
 {- Generates a normal command -}
 command :: String -> String -> [CommandSeek] -> String -> Command
@@ -98,17 +100,22 @@ notBareRepo a = do
 isBareRepo :: Annex Bool
 isBareRepo = fromRepo Git.repoIsLocalBare
 
+numCopies :: FilePath  -> Annex (Maybe Int)
+numCopies file = readish <$> checkAttr "annex.numcopies" file
+
 {- Used for commands that have an auto mode that checks the number of known
  - copies of a key.
  -
  - In auto mode, first checks that the number of known
  - copies of the key is > or < than the numcopies setting, before running
  - the action. -}
-autoCopies :: Key -> (Int -> Int -> Bool) -> Maybe Int -> CommandStart -> CommandStart
-autoCopies key vs numcopiesattr a = Annex.getState Annex.auto >>= auto
+autoCopies :: FilePath -> Key -> (Int -> Int -> Bool) -> (Maybe Int -> CommandStart) -> CommandStart
+autoCopies file key vs a = do
+	numcopiesattr <- numCopies file
+	Annex.getState Annex.auto >>= auto numcopiesattr
 	where
-		auto False = a
-		auto True = do
+		auto numcopiesattr False = a numcopiesattr
+		auto numcopiesattr True = do
 			needed <- getNumCopies numcopiesattr
 			(_, have) <- trustPartition UnTrusted =<< Remote.keyLocations key
-			if length have `vs` needed then a else stop
+			if length have `vs` needed then a numcopiesattr else stop