Initial setup for sharing scripts

[AlvaroBrey]

Inspired by #17

get_branch.sh is meant to replace getBranchName.sh and getBranchBase.sh.

This requires some stuff to be setup in the CI environment:

• GITHUB_TOKEN: self-explanatory, we should use the default ${{ secrets.GITHUB_TOKEN }} on Github, and a PAT on Drone.
• GITHUB_REPOSITORY: example is nextcloud/android (including org). Can be retrieved from Github's env ( ${{ github.repository}}) and also Drone's ($DRONE_REPO)

These are env variables and not shell arguments because they are intended to be used in code common to all scripts (currently only lib.sh), so we won't need to pass them around so much.

[timkrueger]

That is great 👍

[AlvaroBrey]

Marked as draft as I have now become aware of two incompatible uses of getBranchName and getBranchBase. For PRs, the first returns the head ref, and the latter the base ref. So I must look better into how they're used before attempting to make them common like this.

[tobiasKaminsky]

If you need help, just ping me :)