devtools: Auto-set branch to merge to in github-merge

As we are already using the API to retrieve the pull request
title, also retrieve the base branch.

This makes sure that pull requests for 0.12 automatically end up in
0.12, and pull requests for master automatically end up in master,
and so on.

It is still possible to override the branch from the command line
or using the githubmerge.branch git option.