I thought sub-builds and matrix builds were not yet supported?

Also, it's probably best to return an error when the label cannot be found in the DB (currently builds[0] is used).  Also, I don't think it involves an extra DB roundtrip to use GetBuildSlug:

```
var build model.Build
if labl != "" {
  builds, err := database.ListBuilds(commit.ID)
  if err != nil {
    return err
  } else if len(builds) == 0 {
    return fmt.Errorf("Failed to find any builds for commit id %d", commit.ID)
  }
  build = builds[0]
} else {
  var err error
  build, err = database.GetBuildSlug(labl, commit.ID)
  if err != nil {
    return err
  } else if len(builds) == 0 {
    return fmt.Errorf("Failed to find any builds for commit id %d, label %s", commit.ID, labl)
  }
}
```