git: errors.New() doesn't need formatting directives

Small fix for a broken error message:

```
<dgryski@kaepora[go-pushbullet]> gh pull-request -d dgryski:api-host-update
2013/12/10 21:41:16 fatal: Can't load rev-list for %smaster...dgryski:api-host-update
```

(As a side note, that above command was how I always submitted pull-requests, so I was surprised to see it throw an error.  Does this need more investigating?)