Add the update-prs GitHub Action

The idea is to take over the job of the "Update GitGitGadget's PRs"
Azure Pipeline over at
https://dev.azure.com/gitgitgadget/git/_build?definitionId=2.

This will obviously require a GitHub workflow, which is added in
gitgitgadget-workflows/gitgitgadget-workflows#1

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

Author: dscho

update-prs/action.yml

@@ -0,0 +1,19 @@
+name: 'Update the Pull Requests'
+description: 'Updates the Pull Requests in response to upstream commits'
+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
+runs:
+  using: 'node20'
+  main: './index.js'
+branding:
+  icon: 'git-commit'
+  color: 'orange'

update-prs/index.js

@@ -0,0 +1,14 @@
+async function run() {
+  const { CIHelper } = await import("../dist/index.js")
+
+  const ci = new CIHelper()
+
+  await ci.setupGitHubAction({
+    needsUpstreamBranches: true,
+  })
+  await ci.updateOpenPrs()
+  await ci.updateCommitMappings()
+  await ci.handleOpenPRs()
+}
+
+run()