add ExportTree table to export db

New table needed to look up what filenames are used in the currently
exported tree, for reasons explained in export.mdwn.

Also, added smart constructors for ExportLocation and ExportDirectory to
make sure they contain filepaths with the right direction slashes.

And some code refactoring.

This commit was sponsored by Francois Marier on Patreon.

Database/Types.hs

@@ -1,6 +1,6 @@
 {- types for SQL databases
  -
- - Copyright 2015-2016 Joey Hess <id@joeyh.name>
+ - Copyright 2015-2017 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
@@ -16,6 +16,7 @@ import Data.Char
 import Utility.PartialPrelude
 import Key
 import Utility.InodeCache
+import Git.Types (Ref(..))
 
 -- A serialized Key
 newtype SKey = SKey String
@@ -93,3 +94,21 @@ fromSFilePath (SFilePath s) = s
 
 derivePersistField "SFilePath"
 
+-- A serialized Ref
+newtype SRef = SRef Ref
+
+-- Note that Read instance does not work when used in any kind of complex
+-- data structure.
+instance Read SRef where
+	readsPrec _ s = [(SRef (Ref s), "")]
+
+instance Show SRef where
+	show (SRef (Ref s)) = s
+
+derivePersistField "SRef"
+
+toSRef :: Ref -> SRef
+toSRef = SRef
+
+fromSRef :: SRef -> Ref
+fromSRef (SRef r) = r