gitlab-rulez: Increase the number of branches retrieved

The branches listing API is paginated, even if the documentation does not say anything about it:

https://gitlab.com/gitlab-org/gitlab/issues/22976

For this reason, the reply was capped to 20 results by default. It turns out that some repositories just went above that threshold, so the a new branch was not returned even if it was actually available on the repository.

Increase the items-per-page number and hope nobody will create more than 100 branches in a repository. :P