Upgrading Git to 1.9.0 breaks GitDownloadStrategy#checkout_submodules

The end result is that installing [git-hg](https://github.com/Homebrew/homebrew-headonly/blob/c8cc981cf51a5c04815cf689c18af87244337b23/git-hg.rb) resulted in the `fast-export` submodule installed to `/usr/local/Cellar/git-hg/HEAD/$path` (instead of `/usr/local/Cellar/git-hg/HEAD/fast-export`

```
/L/C/H/git-hg--git> brew irb
==> Interactive Homebrew Shell
Example commands available with: brew irb --help
>> system("git", "submodule", "foreach", "--recursive", "echo", "$name", "`git rev-parse HEAD`")
Entering 'fast-export'
$name `git rev-parse HEAD`
=> true
>> system("git", "submodule", "foreach", "--recursive", "echo $name `git rev-parse HEAD`")
Entering 'fast-export'
fast-export aaccfba9a0cfcfdd4db1e629b50d65b1c479b64f
=> true
>>
```