fix(git): throw config error if branch is blocking (#9341)

lib/util/git/index.ts

@@ -703,6 +703,13 @@ export async function commitFiles({
     return commit;
   } catch (err) /* istanbul ignore next */ {
     checkForPlatformFailure(err);
+    if (err.message.includes(`'refs/heads/renovate' exists`)) {
+      const error = new Error(CONFIG_VALIDATION);
+      error.configFile = 'None';
+      error.validationError = 'An existing branch is blocking Renovate';
+      error.validationMessage = `Renovate needs to create the branch "${branchName}" but is blocked from doing so because of an existing branch called "renovate". Please remove it so that Renovate can proceed.`;
+      throw error;
+    }
     if (
       err.message.includes(
         'refusing to allow a GitHub App to create or update workflow'