Select branch when pushing to opendbc fork

[energee]

Suggest pulling default branch or allowing current dbc branch to be selectable.

[jyoung8607]

+1, I like that we can push directly to GitHub, but the fact it always-and-only pushes to master makes a mess out of your fork's master and puts extra steps in the workflow of upstreaming updates (which is, after all, the end goal of using this tool).