Changing the struct GitBranch to use a default constructor and fields

[StanleyGoldman]

The current constructor for this struct prevents us from creating a default value for the struct itself.

Unity/src/GitHub.Api/Git/GitBranch.cs

Lines 21 to 28 in 2f38e6a

	public GitBranch(string name, string tracking, bool active)
	{
	Guard.ArgumentNotNullOrWhiteSpace(name, "name");
	this.name = name;
	this.tracking = tracking;
	this.active = active;
	}

[StanleyGoldman]

I realize you use this interface in #151

[shana]

Just remembered we can do public static GitBranch Default = new GitBranch { }; to leverage the default struct constructor, it should work well enough. Nevermind I'm being dumb we can just use the default constructor

[StanleyGoldman]