ci/github-script/prepare: init from actions/get-merge-commit

This just moves the code over to ci/github-script to make it easy to
test and iterate on locally.

The name `prepare` is chosen, because the script will be extended with
the other steps from "PR / prepare" next.

ci/github-script/prepare.js

@@ -0,0 +1,46 @@
+module.exports = async function ({ github, context, core }) {
+  for (const retryInterval of [5, 10, 20, 40, 80]) {
+    console.log("Checking whether the pull request can be merged...")
+    const prInfo = (await github.rest.pulls.get({
+      owner: context.repo.owner,
+      repo: context.repo.repo,
+      pull_number: context.payload.pull_request.number
+    })).data
+
+    if (prInfo.state != 'open') throw new Error ("PR is not open anymore.")
+
+    if (prInfo.mergeable == null) {
+      console.log(`GitHub is still computing whether this PR can be merged, waiting ${retryInterval} seconds before trying again...`)
+      await new Promise(resolve => setTimeout(resolve, retryInterval * 1000))
+      continue
+    }
+
+    let mergedSha, targetSha
+
+    if (prInfo.mergeable) {
+      console.log("The PR can be merged.")
+
+      mergedSha = prInfo.merge_commit_sha
+      targetSha = (await github.rest.repos.getCommit({
+        owner: context.repo.owner,
+        repo: context.repo.repo,
+        ref: prInfo.merge_commit_sha
+      })).data.parents[0].sha
+    } else {
+      console.log("The PR has a merge conflict.")
+
+      mergedSha = prInfo.head.sha
+      targetSha = (await github.rest.repos.compareCommitsWithBasehead({
+        owner: context.repo.owner,
+        repo: context.repo.repo,
+        basehead: `${prInfo.base.sha}...${prInfo.head.sha}`
+      })).data.merge_base_commit.sha
+    }
+
+    console.log(`Checking the commits:\nmerged:${mergedSha}\ntarget:${targetSha}`)
+    core.setOutput('mergedSha', mergedSha)
+    core.setOutput('targetSha', targetSha)
+    return
+  }
+  throw new Error("Not retrying anymore. It's likely that GitHub is having internal issues: check https://www.githubstatus.com.")
+}