ci: skip GitHub workflow runs for already-tested commits/trees

When pushing a commit that has already passed a CI or PR build
successfully, it makes sense to save some energy and time and skip the
new build.

Let's teach our GitHub workflow to do that.

For good measure, we also compare the tree ID, which is what we actually
test (the commit ID might have changed due to a reworded commit message,
which should not affect the outcome of the run).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.github/workflows/main.yml

@@ -9,7 +9,7 @@ jobs:
   ci-config:
     runs-on: ubuntu-latest
     outputs:
-      enabled: ${{ steps.check-ref.outputs.enabled }}
+      enabled: ${{ steps.check-ref.outputs.enabled }}${{ steps.skip-if-redundant.outputs.enabled }}
     steps:
       - name: try to clone ci-config branch
         run: |
@@ -34,6 +34,43 @@ jobs:
             enabled=no
           fi
           echo "::set-output name=enabled::$enabled"
+      - name: skip if the commit or tree was already tested
+        id: skip-if-redundant
+        uses: actions/github-script@v3
+        if: steps.check-ref.outputs.enabled == 'yes'
+        with:
+          github-token: ${{secrets.GITHUB_TOKEN}}
+          script: |
+            // Figure out workflow ID, commit and tree
+            const { data: run } = await github.actions.getWorkflowRun({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              run_id: context.runId,
+            });
+            const workflow_id = run.workflow_id;
+            const head_sha = run.head_sha;
+            const tree_id = run.head_commit.tree_id;
+
+            // See whether there is a successful run for that commit or tree
+            const { data: runs } = await github.actions.listWorkflowRuns({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              per_page: 500,
+              status: 'success',
+              workflow_id,
+            });
+            for (const run of runs.workflow_runs) {
+              if (head_sha === run.head_sha) {
+                core.warning(`Successful run for the commit ${head_sha}: ${run.html_url}`);
+                core.setOutput('enabled', ' but skip');
+                break;
+              }
+              if (tree_id === run.head_commit.tree_id) {
+                core.warning(`Successful run for the tree ${tree_id}: ${run.html_url}`);
+                core.setOutput('enabled', ' but skip');
+                break;
+              }
+            }
 
   windows-build:
     needs: ci-config