Patch 'unix:filemode' property as 'svn:executable' in SVN

Summary: Patch created on Git contains 'unix:filemode' property which is useless in SVN.

Test Plan: Exported a bundle changing a file to executable in Git, applied it in SVN, verified that the file is executable.

Reviewers: epriestley

Reviewed By: epriestley

CC: ptarjan, aran, Korvin, digoangeline

Differential Revision: https://secure.phabricator.com/D3554