R1:2949905c045b

Fetch and discover all Git ref types, not just branches

Summary:
Ref T9028. Fixes T6878. Currently, we only fetch and discover branches. This is fine 99% of the time but sometimes commits are pushed to just a tag, e.g.:

```
git checkout <some hash>
nano file.c
git commit -am '...'
git tag wild-wild-west
git push origin wild-wild-west
```

Through a similar process, commits can also be pushed to some arbitrary named ref (we do this for staging areas).

With the current rules, we don't fetch tag refs and…