`brew install --HEAD gh` fails

It looks like the formula doesn't actually build from source but installs a binary. That fails when fetching from source.

```
❯❯❯ brew install --HEAD -v gh
==> Cloning https://github.com/jingweno/gh.git
git --git-dir /Library/Caches/Homebrew/gh--git/.git status -s
Updating /Library/Caches/Homebrew/gh--git
git config remote.origin.url https://github.com/jingweno/gh.git
git config remote.origin.fetch +refs/heads/master:refs/remotes/origin/master
git fetch origin
git checkout -f master
Already on 'master'
git reset --hard origin/HEAD
HEAD is now at 5dbe97a Don't use we
git submodule update --init
git reset --hard origin/HEAD
HEAD is now at 5dbe97a Don't use we
git checkout-index -a -f --prefix=/private/tmp/gh-8BOa/
git submodule --quiet foreach --recursive git checkout-index -a -f --prefix=/private/tmp/gh-8BOa/$path/
Error: gh does not exist
```

A formula's `url` shouldn't point to a binary distribution but to a copy of the source. Binaries should be distributed as [bottles](https://github.com/Homebrew/homebrew/wiki/Bottles). However, I'm not clear on how to distribute bottles for taps.

It might be time for `gh`'s formula to get into Homebrew proper anyhow.