When observing a repository, switch to "importing" mode on a large discovery in an empty repository

Summary:
Ref T10923. Fixes T9554.

When hosting a repository, we currently have a heuristic that tries to detect when you're doing an initial import: if you push more than 7 commits to an empty repository, it counts as an import and we disable mail/feed/etc.

Do something similar for observed repositories: if the repository is empty and we discover more than 7 commits, switch to import mode until we catch up.

This should align behavior with user expectation more often when juggling hosted vs imported repositories.

Test Plan:

• Created a new hosted repository.
• Activated it and allowed it to fully import.
• Added an "Observe URI".
• Saw it automatically drop into "Importing" mode until the import completed.
• Swapped it back to hosted mode.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9554, T10923

Differential Revision: https://secure.phabricator.com/D15877