use TopFilePath for associated files

Fixes several bugs with updates of pointer files. When eg, running
git annex drop --from localremote
it was updating the pointer file in the local repository, not the remote.
Also, fixes drop ../foo when run in a subdir, and probably lots of other
problems. Test suite drops from ~30 to 11 failures now.

TopFilePath is used to force thinking about what the filepath is relative
to.

The data stored in the sqlite db is still just a plain string, and
TopFilePath is a newtype, so there's no overhead involved in using it in
DataBase.Keys.

Git/FilePath.hs

@@ -13,9 +13,8 @@
 {-# LANGUAGE CPP #-}
 
 module Git.FilePath (
-	TopFilePath,
+	TopFilePath(..),
 	fromTopFilePath,
-	getTopFilePath,
 	toTopFilePath,
 	asTopFilePath,
 	InternalGitPath,