When creating foreign stubs during SVN import, mark them imported

Summary: See <https://github.com/facebook/phabricator/issues/501>. I think the issue here is that we created a foreign stub for commit `X-1`, probably because commit `X` was created by running `svn cp y x`.

Test Plan: I'll write a separate test for this before I land it. Huge pain to test.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D8133