R1:3607bd487cc0

Survive pull/discover for hosted repositories in all VCSes

Summary:
Hosted repositories only sometimes survive the pull/discover phases right now, due to issues like:

- Pull tries to `git clone`, but should `git init`.
- Mercurial doesn't handle empty repositories with on branches.
- SVN tries to connect to an invalid remote.
- None of them set the INIT repo flag correctly, so status doesn't get updated properly in the UI.

Fix all this stuff.

Test Plan:
- For each of Git, SVN and Mercurial…