Fix resolution of commits in SVN repositories without callsigns

Summary:
Fixes T10721. When trying to load commits by identifier, we would take some bad pathways in Subversion if the repository had no callsign and end up missing the commits.

Fix this logic so it works for either callsigns (e.g., if passed rXyyy) or with PHIDs if passed repositories.

Test Plan:

• Viewed SVN commit in a Subversion repository with no callsign.
• Added a callsign, looked at it again.
• Viewed non-SVN commits in callsign and non-callsign repositories.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10721

Differential Revision: https://secure.phabricator.com/D15607