fix(github): respect automergeStrategy with platform automerge (#44563)

* fix(github): respect automerge strategy with platform automerge

* docs(automerge): clarify GitHub platform strategy

* docs(automerge): generalize platform strategy behavior

Author: jetersen

docs/usage/configuration-options.md

@@ -303,6 +303,8 @@ If possible, Renovate follows the merge strategy set on the platform itself for
 If you've set `automerge=true` and `automergeType=pr` for any of your dependencies, then you may choose what automerge strategy Renovate uses by setting the `automergeStrategy` config option.
 If you're happy with the default behavior, you don't need to do anything.
 
+On supported platforms, `automergeStrategy` also applies when using platform-native automerge.
+
 You may choose from these values:
 
 - `auto`, Renovate decides how to merge
@@ -4067,6 +4069,8 @@ If enabled Renovate will pin Docker images or GitHub Actions by means of their S
 
 If you have enabled `automerge` and set `automergeType=pr` in the Renovate config, then leaving `platformAutomerge` as `true` speeds up merging via the platform's native automerge functionality.
 
+Where supported, platform-native automerge uses [`automergeStrategy`](#automergestrategy) to select the merge method.
+
 On Bitbucket Server, GitHub and GitLab, Renovate re-enables the PR for platform-native automerge whenever it's rebased.
 
 `platformAutomerge` will configure PRs to be merged after all (if any) branch policies have been met.

lib/modules/platform/github/index.spec.ts

@@ -4174,6 +4174,36 @@ describe('modules/platform/github/index', () => {
         ]);
       });
 
+      it('should use the configured automerge strategy', async () => {
+        const scope = await mockScope();
+        scope.post('/graphql').reply(200, graphqlAutomergeResp);
+
+        const pr = await github.createPr({
+          ...prConfig,
+          platformPrOptions: {
+            usePlatformAutomerge: true,
+            automergeStrategy: 'rebase',
+          },
+        });
+
+        expect(pr).toMatchObject({ number: 123 });
+        expect(httpMock.getTrace()).toMatchObject([
+          graphqlGetRepo,
+          restCreatePr,
+          restAddLabels,
+          {
+            ...graphqlAutomerge,
+            graphql: {
+              ...graphqlAutomerge.graphql,
+              variables: {
+                pullRequestId: 'abcd',
+                mergeMethod: 'REBASE',
+              },
+            },
+          },
+        ]);
+      });
+
       it('should handle GraphQL errors', async () => {
         const scope = await mockScope();
         scope.post('/graphql').reply(200, graphqlAutomergeErrorResp);

lib/modules/platform/github/index.ts

@@ -1887,7 +1887,11 @@ async function tryPrAutomerge(
   }
 
   try {
-    const mergeMethod = config.mergeMethod?.toUpperCase() || 'MERGE';
+    const mergeMethod =
+      (
+        mapMergeStartegy(platformPrOptions.automergeStrategy) ??
+        config.mergeMethod
+      )?.toUpperCase() || 'MERGE';
 
     let commitHeadline: string | undefined;
     let commitBody: string | undefined;