R1:f6915a797573

Add a heursitic for initial pushes which are really imports

Summary:
Fixes T7298. There are two ways to import a repository that you want to host, today:

- Create it as "hosted", then push everything to it.
- Create it as "imported", let it import, then switch it to "hosted".
- (Neither of these work with SVN.)

We don't specifically recommend one or the other, although I believe both should work, and most users seem to go with the first one.

In the first workflow, the new empty repository imports…