workflows/get-merge-commit: inline get-merge-commit.sh script as github-script

The reason this was a separate shell script was, that this would be
included in multiple workflows separately. But a while ago this had been
changed to a re-usable workflow, so we can just as well inline the
script.

This also allows us to use actions/github-script, which makes for a much
more readable script than the bash script before.

.github/workflows/get-merge-commit.yml

@@ -22,8 +22,8 @@ jobs:
   resolve-merge-commit:
     runs-on: ubuntu-24.04-arm
     outputs:
-      mergedSha: ${{ steps.merged.outputs.mergedSha }}
-      targetSha: ${{ steps.merged.outputs.targetSha }}
+      mergedSha: ${{ fromJSON(steps.merged.outputs.result).mergedSha }}
+      targetSha: ${{ fromJSON(steps.merged.outputs.result).targetSha }}
       systems: ${{ steps.systems.outputs.systems }}
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
@@ -33,24 +33,45 @@ jobs:
 
       - name: Check if the PR can be merged and get the test merge commit
         id: merged
-        env:
-          GH_TOKEN: ${{ github.token }}
-          GH_EVENT: ${{ github.event_name }}
-        run: |
-          case "$GH_EVENT" in
-            push)
-              echo "mergedSha=${{ github.sha }}" >> "$GITHUB_OUTPUT"
-              ;;
-            pull_request*)
-              if commits=$(base/ci/get-merge-commit.sh ${{ github.repository }} ${{ github.event.number }}); then
-                echo -e "Checking the commits:\n$commits"
-                echo "$commits" >> "$GITHUB_OUTPUT"
-              else
-                # Skipping so that no notifications are sent
-                echo "Skipping the rest..."
-              fi
-              ;;
-          esac
+        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 {
+                throw new Error("The PR has a merge conflict. Skipping the rest...")
+              }
+            }
+            throw new Error("Not retrying anymore. It's likely that GitHub is having internal issues: check https://www.githubstatus.com.")
 
       - name: Load supported systems
         id: systems