Build: fix Grunt build-release/commit on SVN checkouts.

Old bbPress, which is a SVN external, was not being accounted for in the tasks' working directories/files.
This was causing most of our Grunt build commands to fail when run on a SVN checkout.

Fixes #6015


git-svn-id: https://buddypress.svn.wordpress.org/trunk@9149 cdf35c40-ae34-48e0-9cc9-0c9da1808c22