Add CI for automatic PHP8 branch updates

[rugk]

Adds a simple CI for pushing the master branches changes to the php8 branch.

Useful/discussed for #707

Unfortunately we can only really test it if it is merged, of course…

[elrido]

Thank you, let's give this a try.

[rugk]

Okay huh it fails with:

error: pathspec 'php8' did not match any file(s) known to git

…which is a little strange given this is the correct branch name…

[elrido]

Hm, I'd bet on the actions/checkout trying to be clever and doing a depth=1 checkout, which would not include any other branches.

Looking at the docs, maybe we could do something like this?

- uses: actions/checkout@v2
  with:
    # directly checkout the php8 branch, remove the next action to switch to this branch
    ref: php8
    # Number of commits to fetch. 0 indicates all history for all branches and tags.
    # Default: 1
    fetch-depth: 0

[rugk]

Thanks a lot, applied that in #847