Set "importing" flag on repositories created via API

Summary: Ref T6516. We incorrectly fail to set this flag on repositories created via Conduit, which activates too many actions on old commits.

Test Plan:

• Created a new repository via Conduit, verified it was "importing" after creation.
• Created a new repostiory via web UI, verified it was "importing" after creation.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6516

Differential Revision: https://secure.phabricator.com/D11964