fileRef: make paths relative and simplified

Fix behavior of several commands, including reinject, addurl, and rmurl
when given an absolute path to an unlocked file, or a relative path that
leaves and re-enters the repository.

To avoid slowing down all the cases where the paths are already ok
with an unncessary call to getCurrentDirectory, put in an optimisation
in relPathCwdToFile. That will probably also speed up other parts of
git-annex by some small amount, but I have not benchmarked.

Note that I did not convert branchFileRef, because it seems likely that
it will be used with a file that is not provided by the user, so is already
in a sane format. This is certainly true for the way git-annex uses it,
though maybe arguable to the extent Git.Ref is a reusable library.

Command/Smudge.hs

@@ -103,7 +103,7 @@ clean file = do
 				getMoveRaceRecovery k file
 				liftIO $ L.hPut stdout b
 		Nothing -> do
-			let fileref = Git.Ref.fileRef file
+			fileref <- liftIO $ Git.Ref.fileRef file
 			indexmeta <- catObjectMetaData fileref
 			oldkey <- case indexmeta of
 				Just (_, sz, _) -> catKey' fileref sz