rPe910c76e65e5

Add "Fetch Rules" to observed Git repositories

Summary:
Depends on D20421. Ref T13277. I'd generally like to move away from "Track Only".

Some of the use cases for "Track Only" (or adjacent to "Track Only") are better resolved with "Fetch Rules" -- basically, rules to fetch only some subset of refs from the observed remote.

Add configurable "Fetch Rules" for Git repositories. For example, if you only want to fetch `master`, you can now speify:

```
refs/heads/master
```

If you only want to fetch…