Fix an issue where known Subversion commits are incorrectly shown as "Discovering..."

Summary:
Ref T13552. The behavior of "RepositoryQuery" with ambiguous identifiers under "withRepositoryPHIDs()" is tricky. This leads to failure to load commits in Subversion in some cases.

Use "withRepository()", which gives us the correct identifier resolution behavior.

Test Plan: Viewed a subversion repository history in Diffusion, saw commit details after change.

Maniphest Tasks: T13552

Differential Revision: https://secure.phabricator.com/D21469