split off versionedExport from appendonly

S3 uses versionedExport, while GitLFS uses appendonly.

This is groundwork for later changes.

Remote/WebDAV.hs

@@ -100,6 +100,7 @@ gen r u rc gc rs = do
 				, retrieveExport = retrieveExportDav hdl
 				, checkPresentExport = checkPresentExportDav hdl this
 				, removeExport = removeExportDav hdl
+				, versionedExport = False
 				, removeExportDirectory = Just $
 					removeExportDirectoryDav hdl
 				, renameExport = renameExportDav hdl