Add the handle-pr-push GitHub Action

 This GitHub Action is designed to implement the functionality that is
currently handled when pushing to PRs by these Azure Pipelines:

- GitGitGadget PR Handler:
  https://dev.azure.com/gitgitgadget/git/_build?definitionId=3

- GitGitGadget PR Handler (git):
  https://dev.azure.com/gitgitgadget/git/_build?definitionId=13

- GitGitGadget PR Handler (dscho):
  https://dev.azure.com/gitgitgadget/git/_build?definitionId=12

This Action will be used in a GitHub workflow that will replace those
trusty Azure Pipelines we used for so long. This workflow lives in a
different org, for full details see
gitgitgadget-workflows/gitgitgadget-workflows#1.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

handle-pr-push/action.yml

@@ -0,0 +1,22 @@
+name: 'Handle PR Pushes'
+description: 'Handles when a PR was pushed'
+author: 'Johannes Schindelin'
+inputs:
+  pr-repo-token:
+    description: 'The access token to work on the repository that holds PRs and state'
+    required: true
+  upstream-repo-token:
+    description: 'The access token to work on PRs in the upstream repository'
+    required: true
+  test-repo-token:
+    description: 'The access token to work on PRs in the test repository'
+    required: true
+  pr-url:
+    description: 'The URL of the PR that was pushed'
+    required: true
+runs:
+  using: 'node20'
+  main: './index.js'
+branding:
+  icon: 'git-commit'
+  color: 'orange'

handle-pr-push/index.js

@@ -0,0 +1,11 @@
+async function run() {
+  const { CIHelper } = await import("../dist/index.js")
+
+  const ci = new CIHelper()
+  const { owner, prNumber } = ci.parsePRURLInput()
+
+  await ci.setupGitHubAction()
+  await ci.handlePush(owner, prNumber)
+}
+
+run()