Create and synchronize a PR in a dependent repository

# Hacking

The test environment consists of the following

* A forgejo instance with a runner
* An unprivileged user user1
* The repository user1/originrepo
  * contains a pull_request workflow using cascading-pr that targets user2/destinationrepo
  * contains a script that will modify user2/destinationrepo
  * a PR from branch1 to main
* The repository user1/cascading-pr with the action under test
* An unprivileged user user2
* The repository user2/destinationrepo

## testing an update on the action

* run `tests/run.sh --debug` once so all is in place
* commit changes to the files that are in the cascading-pr action
  (action.yml, cascading-pr.sh etc.)
* push the modified action to `user1/cascading-pr`
* visit $url/user1/originrepo/actions/runs/1 and click re-run

## interactive debugging

Following the steps below recreate the same environment as the
integration workflow locally. It is helpful for forensic analysis when
something does not run as expected and the error displayed are unclear.

To help with the development loop all steps are idempotent and
running `tests/run.sh --debug` multiple times must succeed.

Individual steps can be run independendely by using the name of the function.
For instance:

* `tests/run.sh --debug create_pull_request` will only call the `create_pull_request`
  function found in `tests/run.sh` to (re)create the pull request in `user1/originrepo`.
* `./cascading-pr.sh --debug --origin-url ... upsert_branch` will only call the `upsert_branch`
  function found in `cascading-pr.sh`.

## directories

The `tests/run.sh` script stores all its files in
`/tmp/cascading-pr-test`. The temporary directories created by
`cascading-pr.sh` are disposed of when the script ends.

## logging

If `--debug` is used a full debug log is displayed, very complete and
very verbose. Otherwise it is stashed in a temporary file and only
displayed if an error happens.

## snippets for copy/pasting

```sh
git clone https://code.forgejo.org/actions/setup-forgejo
export PATH=$(pwd)/setup-forgejo:$PATH
git clone https://code.forgejo.org/actions/cascading-pr
cd cascading-pr
forgejo-curl.sh logout
forgejo-runner.sh teardown
forgejo.sh teardown
forgejo.sh setup root admin1234 codeberg.org/forgejo/forgejo 1.21
forgejo-runner.sh setup
url=http://$(cat forgejo-ip):3000
firefox $url
tests/run.sh --debug
tests/run.sh --debug create_pull_request
./cascading-pr.sh --debug --origin-url "$url" --origin-repo "user1/originrepo" --origin-token "$(cat /tmp/cascading-pr-test/user1/repo-token)" --origin-pr 1 --destination-url "$url" --destination-repo "user2/destinationrepo" --destination-token "$(cat /tmp/cascading-pr-test/user2/repo-token)" --destination-branch "main" --update "upgraded" run
```