reorder repo parameters last

Many functions took the repo as their first parameter. Changing it
consistently to be the last parameter allows doing some useful things with
currying, that reduce boilerplate.

In particular, g <- gitRepo is almost never needed now, instead
use inRepo to run an IO action in the repo, and fromRepo to get
a value from the repo.

This also provides more opportunities to use monadic and applicative
combinators.

Backend/SHA.hs

@@ -98,9 +98,8 @@ keyValueE size file = keyValue size file >>= maybe (return Nothing) addE
 {- A key's checksum is checked during fsck. -}
 checkKeyChecksum :: SHASize -> Key -> Annex Bool
 checkKeyChecksum size key = do
-	g <- gitRepo
 	fast <- Annex.getState Annex.fast
-	let file = gitAnnexLocation g key
+	file <- fromRepo $ gitAnnexLocation key
 	present <- liftIO $ doesFileExist file
 	if not present || fast
 		then return True