fix(dev-infra): merge script should not always require full repo permissions (#37718)

We recently added OAuth scope checking to the dev-infra Git client
and started leveraging it for the merge script. We set the `repo` scope
as required for running the merge script. We can loosen this requirement
as in the Angular org where the script is consumed, only pull requests on
public repositories are merged through the script.

This should help with reducing the risk with compromised tokens as no
access had to be granted on `repo:invite`, `repo_deployment` etc.

PR Close #37718

dev-infra/utils/config.ts

@@ -21,6 +21,8 @@ export interface GitClientConfig {
   name: string;
   /** If SSH protocol should be used for git interactions. */
   useSsh?: boolean;
+  /** Whether the specified repository is private. */
+  private?: boolean;
 }
 
 /**