Retrieve commit payload from message body

Backported from 99ec266.

The legacy MantisBT GitHub Service sends the push's payload via
eponymous form variable. With the retirement of this service and its
replacement by webhooks, the payload is now contained in the body.

Fixes #292

Author: dregad

SourceGithub/SourceGithub.php

@@ -248,9 +248,14 @@ private function api_json_url( $p_repo, $p_url, $p_member = null ) {
 	}
 
 	public function precommit() {
+		# Legacy GitHub Service sends the payload via eponymous form variable
 		$f_payload = gpc_get_string( 'payload', null );
 		if ( is_null( $f_payload ) ) {
-			return;
+			# If empty, retrieve the webhook's payload from the body
+			$f_payload = file_get_contents( 'php://input' );
+			if ( is_null( $f_payload ) ) {
+				return;
+			}
 		}
 
 		if ( false === stripos( $f_payload, 'github.com' ) ) {