Urls that contain tree anywhere in the filepath break branch parsing

[lehnerm]

The occurrence of tree in the url is checked without respect to position (see index.js), which leads to an error in parsing the correct branch from the url.

Example case:
https://github.com/contentful/extensions/blob/master/samples/content-tree/extension.json

Will return:
branch: 'extension.json'

Should return:
branch: 'master'