R1:71a97d8af56b

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…