Disable merging to unsupported branches on origin

In our release meeting in Brussels, we talked about disabling merges to unsupported branches: https://trac.torproject.org/projects/tor/wiki/org/meetings/2019BrusselsNetworkTeam/Notes/StableMaintainer#ActionItems

In particular, we should only allow merges to current release and maint branches, and master.

Here is a list of current releases (prefixed with release- and maint-):

• 0.2.9 (LTS)
• 0.3.3 (expires 28 February 2019)
• 0.3.4
• 0.3.5 (LTS)
• 0.4.0
• 0.4.1 (in git, 0.4.1 is the master branch at the moment)

We should also allow:

• master

https://trac.torproject.org/projects/tor/wiki/org/teams/NetworkTeam/CoreTorReleases#Current