Needs more comments
The code is very well written but it's missing comments. Also not having curly braces {} after if statements could cause future problems.
Also, in this file wp-load is loaded. https://github.com/PressLabs/gitium/blob/master/gitium/gitium-webhook.php Why not have a hook init and to check for a specific request parameter and if there is one then hook into WP and output stuff.
WP can be in a different directory, not just DOCUMENT_ROOT
Thanks for review!
The gitium-webhook.php is supposed to be completely separated from WP, in order to be ready to recover it from git, in case you break the site. About comments, I put some, but still needs more. The format of the code is WP std. and I will update it as soon as posible. About WP dir I will investigate it. Thanks.
Cool. Great job. Keep working on this.