Fix lookup of commits in Subversion

Summary:
Fixes T7122. The way this query works is a little surprising:

• If executed as withRepositoryIDs(...), it assumes you are passing one or more repository IDs, so it will never resolve ambiguous identifiers (e.g., "123" instead of "rSVN123").
• If executed as withRepository(...), it knows you are passing exactly one repository and will use that to imply context and resolve these identifiers correctly.

This isn't very obvious from the API, but I'm not sure how to make it more clear.

(Making withRepositoryIDs() do the withRepository(...) thing if only one ID was passed in would mean its behavior varied if you passed 1 vs 2 repository IDs, which seems worse / morse surprising.)

Test Plan: Various subversion UIs no longer fail to look up commits.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: mormegil, epriestley

Maniphest Tasks: T7122

Differential Revision: https://secure.phabricator.com/D11645