R1:d28ddc21a550

Don't error when trying to mirror or observe an empty repository

Summary:
Fixes T5965.

Fixes two issues:

- Observing an empty repository could write a warning to the log.
- Mirroring an empty repository to a remote could fail.

For observing:

If newly-created with `git init --bare`, `git ls-remote` will
return the empty string. Properly return an empty set of refs, rather
than attempting to parse the single "line" that is produced by
splitting that on newlines:

```
[2018-01-23 18:47:00] ERROR 8…