Set full path for DiffusionRepositoryPath in svn diffusion browse query

Summary:
The full path field of the DiffusionRepositoryPath object is used by the
DiffusionBrowseController when viewing a directory with a readme file, so
we should set this field.

Test Plan: loaded a directory containing a readme in a svn repo

Reviewers: epriestley, jungejason, emiraga

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D2045