Make sure hub build scripts have access to Git data

The hub build script depends on having Git data to determine a version.  With the existing script, the HEAD hub binary returns a bad version number:

```
$ brew install --HEAD hub
==> Cloning https://github.com/github/hub.git
Updating /Library/Caches/Homebrew/hub--git
==> Checking out branch master
==> script/build
==> Caveats
Bash completion has been installed to:
  /usr/local/etc/bash_completion.d

zsh completion has been installed to:
  /usr/local/share/zsh/site-functions
==> Summary
🍺  /usr/local/Cellar/hub/HEAD: 6 files, 9.4M, built in 2 seconds

$ hub version
git version 2.2.1
hub version -extld=clang
```

WIth this change:
```
$ hub version
git version 2.2.1
hub version 2.2.0-preview1-95-gcc56a71
```

/cc github/hub#745