fix: delete blocking branch before attempting create

lib/platform/github/index.js

@@ -1140,9 +1140,17 @@ async function createBranch(branchName, sha) {
     options.token = config.forkToken;
   }
   try {
-    config.branchList.push(branchName);
+    // istanbul ignore if
+    if (branchName.includes('/')) {
+      const [blockingBranch] = branchName.split('/');
+      if (await branchExists(blockingBranch)) {
+        logger.warn({ blockingBranch }, 'Deleting blocking branch');
+        await deleteBranch(blockingBranch);
+      }
+    }
     logger.debug({ options, branchName }, 'Creating branch');
     await get.post(`repos/${config.repository}/git/refs`, options);
+    config.branchList.push(branchName);
     logger.debug('Created branch');
   } catch (err) /* istanbul ignore next */ {
     logger.warn(