feat: allow push-to-fork to push to sibling repos (#2414)

Fixes #2412.

src/create-pull-request.ts

@@ -84,11 +84,22 @@ export async function createPullRequest(inputs: Inputs): Promise<void> {
       core.info(
         `Checking if '${branchRepository}' is a fork of '${baseRemote.repository}'`
       )
-      const parentRepository =
+      const baseParentRepository = await githubHelper.getRepositoryParent(
+        baseRemote.repository
+      )
+      const branchParentRepository =
         await githubHelper.getRepositoryParent(branchRepository)
-      if (parentRepository != baseRemote.repository) {
+      if (branchParentRepository == null) {
         throw new Error(
-          `Repository '${branchRepository}' is not a fork of '${baseRemote.repository}'. Unable to continue.`
+          `Repository '${branchRepository}' is not a fork. Unable to continue.`
+        )
+      }
+      if (
+        branchParentRepository != baseRemote.repository &&
+        baseParentRepository != branchParentRepository
+      ) {
+        throw new Error(
+          `Repository '${branchRepository}' is not a fork of '${baseRemote.repository}', nor are they siblings. Unable to continue.`
         )
       }
       // Add a remote for the fork

src/github-helper.ts

@@ -101,14 +101,12 @@ export class GitHubHelper {
     }
   }
 
-  async getRepositoryParent(headRepository: string): Promise<string> {
+  async getRepositoryParent(headRepository: string): Promise<string | null> {
     const {data: headRepo} = await this.octokit.rest.repos.get({
       ...this.parseRepository(headRepository)
     })
     if (!headRepo.parent) {
-      throw new Error(
+      return null
-        `Repository '${headRepository}' is not a fork. Unable to continue.`
-      )
     }
     return headRepo.parent.full_name
   }