fixes cargo build command on osx

These changes allows to use servo/rust-http repository as a dependency in Cargo.toml

Here is a sample program:
```rust
extern crate url;
extern crate http;

fn mian() -> () {
    let url: url::Url = Url::parse("http://example.com/example?test").unwrap();
    let request: http::client::RequestWriter = http::client::RequestWriter::new(Get, url).unwrap();
    println!("test = {}", request.url);
}
```
and Cargo.toml which I use:
```yaml
...
[[bin]]
name = "blumenplace-border"
path = "src/main.rs"
[dependencies.url]
git = "https://github.com/servo/rust-url.git"
[dependencies.http]
git = "https://github.com/zolkko/rust-http.git"
```

Beside this, it seems like, Cargo requires that a repository contained a branch named "master". Otherwise there will be following error:
```
cargo build --verbose
    Updating git repository `https://github.com/servo/rust-http.git`
Unable to update https://github.com/servo/rust-http.git

Caused by:
 10 #[phase(plugin, link)] extern crate log;
  Executing `git rev-parse master` failed

Caused by:
  Could not execute process `git rev-parse master` (status=128)
--- stdout
master

--- stderr
fatal: ambiguous argument 'master': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
```