use a subdirectory of annex.dbdir

This allows annex.dbdir to be set globally or always set to the same
value when needed. Each repository uses a subdirectory of it.

Sponsored-by: Dartmouth College's Datalad project

Database/Init.hs

@@ -38,7 +38,7 @@ initDb db migration = do
 	gc <- Annex.getGitConfig
 	top <- parentDir <$> fromRepo gitAnnexDir
 	let tops = case annexDbDir gc of
-		Just topdbdir -> [top, parentDir topdbdir]
+		Just topdbdir -> [top, parentDir (parentDir topdbdir)]
 		Nothing -> [top]
 	liftIO $ do
 		createDirectoryUnder tops tmpdbdir