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.

Command/Migrate.hs

@@ -42,14 +42,13 @@ upgradableKey key = isNothing $ Types.Key.keySize key
 
 perform :: FilePath -> Key -> Backend Annex -> CommandPerform
 perform file oldkey newbackend = do
-	g <- gitRepo
-
 	-- Store the old backend's cached key in the new backend
 	-- (the file can't be stored as usual, because it's already a symlink).
 	-- The old backend's key is not dropped from it, because there may
 	-- be other files still pointing at that key.
-	let src = gitAnnexLocation g oldkey
-	let tmpfile = gitAnnexTmpDir g </> takeFileName file
+	src <- fromRepo $ gitAnnexLocation oldkey
+	tmp <- fromRepo $ gitAnnexTmpDir
+	let tmpfile = tmp </> takeFileName file
 	liftIO $ createLink src tmpfile
 	k <- Backend.genKey tmpfile $ Just newbackend
 	liftIO $ cleantmp tmpfile