Fix repository URI generation for SVN commit identifiers represented with integer types

Summary:
Ref T13588. See PHI2198. Some pathways represent SVN commit identifiers (which are numeric) with integers; these raise an exception in phutil_nonempty_string(), which is strict.

(Ideally, commit identifiers wouldn't have mixed types, but that's a dramatically less practical change.)

Test Plan: Will deploy.

Maniphest Tasks: T13588

Differential Revision: https://secure.phabricator.com/D21852