In the SVN "Change" view, identify the last time a file was modified instead of 404ing

Summary:
If a file was last modified at revision 50 and you look at revision 55, we currently 404. Instead, always identify the last modification.

Also simplify some of the query objects.

Test Plan: Viewed after-modification revisions for several files in SVN.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran, epriestley

Maniphest Tasks: T851

Differential Revision: https://secure.phabricator.com/D2028