A very minor nitpick: my understanding of idiomatic Go style is that we should avoid unnecessary usage of `else` where you're already returning in the : http://golang.org/doc/effective_go.html#if

Would you mind tweaking this commit to the following?

```go
	if herokuGitHost := os.Getenv("HEROKU_GIT_HOST"); herokuGitHost != "" {
		return herokuGitHost
	}
	if herokuHost := os.Getenv("HEROKU_HOST"); herokuHost != "" {
		return herokuHost
	}
```

Although it would add an extra line, I think the result would be slightly more obvious & easier to scan.