oca-copy-branches tool, used to migrate the branches from Launchpad to GitHub

It has already run on all the registered branches, the result is visible on all the OCA repositories.
It copies the branches and the tags in the repositories.

It can be run several times without issues, even on different machines, because `git-remote-bzr` will assign the same SHA on the commits each time. The effect if it runs several time is that branches will be updated from Launchpad.

I also started a guide for the developers having to move their proposal: https://github.com/OCA/maintainers-tools/wiki/How-to-move-a-Merge-Proposal-to-GitHub. Thanks to @eLBati (https://github.com/OCA/maintainers-tools/pull/2) they can use the same command. I think the process to move a merge proposal so if anyone has ideas to cut some steps, they would be very welcome.