Add yet more unit tests for Subversion

Summary: Ref T4327. This adds a subpath/partial repository where we import only a subdirectory, and adds tests for it.

Test Plan: Ran unit tests.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4327

Differential Revision: https://secure.phabricator.com/D8019