workflows/get-merge-commit: move to composite action

We don't need a separate workflow anymore, because we don't need to skip
dependent jobs on failures anymore. The biggest failure mode was
"conflict" previously, but we resolved that on the last commit. The
remaining failure modes are so rare, that it's OK to just fail the jobs
in this case instead of marking them as "skipped". Especially, because
the resolve-merge-commit job would have previously failed anyway.

By moving this to an action we avoid running separate jobs each time we
need the merge commit. This also makes the check list in PRs much
cleaner.

.github/actions/get-merge-commit/action.yml

@@ -0,0 +1,66 @@
+name: Get merge commit
+
+description: 'Checks whether the Pull Request is mergeable and returns two commit hashes: The result of a temporary merge of the head branch into the target branch ("merged"), and the parent of that commit on the target branch ("target"). Handles push events and merge conflicts gracefully.'
+
+outputs:
+  mergedSha:
+    description: "The merge commit SHA"
+    value: ${{ fromJSON(steps.merged.outputs.result).mergedSha }}
+  targetSha:
+    description: "The target commit SHA"
+    value: ${{ fromJSON(steps.merged.outputs.result).targetSha }}
+
+runs:
+  using: composite
+  steps:
+    - id: merged
+      uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea # v7.0.1
+      with:
+        script: |
+          if (context.eventName == 'push') return { mergedSha: context.sha }
+
+          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 ${retryInterval} seconds before trying again...`)
+              await new Promise(resolve => setTimeout(resolve, retryInterval * 1000))
+              continue
+            }
+
+            if (prInfo.mergeable) {
+              console.log("The PR can be merged.")
+
+              const mergedSha = prInfo.merge_commit_sha
+              const targetSha = (await github.rest.repos.getCommit({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                ref: prInfo.merge_commit_sha
+              })).data.parents[0].sha
+
+              console.log(`Checking the commits:\nmerged:${mergedSha}\ntarget:${targetSha}`)
+
+              return { mergedSha, targetSha }
+            } else {
+              console.log("The PR has a merge conflict.")
+
+              const mergedSha = prInfo.head.sha
+              const targetSha = (await github.rest.repos.compareCommitsWithBasehead({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                basehead: `${prInfo.base.sha}...${prInfo.head.sha}`
+              })).data.merge_base_commit.sha
+
+              console.log(`Checking the commits:\nmerged:${mergedSha}\ntarget:${targetSha}`)
+
+              return { mergedSha, targetSha }
+            }
+          }
+          throw new Error("Not retrying anymore. It's likely that GitHub is having internal issues: check https://www.githubstatus.com.")