R1:1b40f7e5408b

Always initialize Git repositories with "git init", never with "git clone"

Summary:
Fixes T13448. We currently "git clone" to initialize repositories, but this will fetch too many refs if "Fetch Refs" is configured.

In modern Phabricator, there's no apparent reason to "git clone"; we can just "git init" instead. This workflow naturally falls through to an update, where we'll do a "git fetch" and pull in exactly the refs we want.

Test Plan:
- Configured an observed repository with "Fetch Refs".