Extract everything after refs/heads/ as branch.

By dropping the front part of the ref, one is able to use branches with deeper
nesting than 1. Since the subproject configuration allows this, it should be
supported by the github helper method.

Example ref spec:
* master
* foo/bar
* foo/bar/baz

For documentation, see (Payload -> ref) at
https://developer.github.com/v3/activity/events/types/#pushevent

weblate/trans/views/api.py

@@ -31,6 +31,7 @@ from weblate.trans.util import get_site_url
 import json
 import weblate
 import threading
+import re
 
 
 BITBUCKET_GIT_REPOS = (
@@ -193,7 +194,7 @@ def github_hook_helper(data):
     # Parse owner, branch and repository name
     owner = data['repository']['owner']['name']
     slug = data['repository']['name']
-    branch = data['ref'].split('/')[-1]
+    branch = re.sub(r'^refs/heads/', '', data['ref'])
 
     # Construct possible repository URLs
     repos = [repo % {'owner': owner, 'slug': slug} for repo in GITHUB_REPOS]