use ByteStrings when reading annex symlinks and pointers

Now there's a ByteString used all the way from disk to Key.

The main complication in this conversion was the use of fromInternalGitPath
in several places to munge things on Windows. The things that used that
were changed to parse the ByteString using either path separator.

Also some code that had read from files to a String lazily was changed
to read a minimal strict ByteString.

Command/DiffDriver.hs

@@ -88,7 +88,7 @@ fixupReq req@(Req {}) =
 	check getfile getmode setfile r = case readTreeItemType (getmode r) of
 		Just TreeSymlink -> do
 			v <- getAnnexLinkTarget' (getfile r) False
-			case fileKey . takeFileName =<< v of
+			case parseLinkTargetOrPointer =<< v of
 				Nothing -> return r
 				Just k -> setfile r <$>
 					withObjectLoc k