fix(dev-infra): local changes check not working (#37489)

Looks like we broke the `hasLocalChanges` check in the git client
when we moved it over from the merge script. The problem is that
we are using `git` in the first argument of `git.run`. That
means that we under-the-hood run `git git <..>`.

This commit fixes that, but also switches to a better variant
for ensuring no local changes because it exits with non-zero
when there are local changes.

PR Close #37489

dev-infra/utils/git.ts

@@ -142,7 +142,7 @@ export class GitClient {
 
   /** Whether the repo has any local changes. */
   hasLocalChanges(): boolean {
-    return !!this.runGraceful(['git', 'status', '--porcelain']).stdout.trim();
+    return this.runGraceful(['diff-index', '--quiet', 'HEAD']).status !== 0;
   }
 
   /** Sanitizes a given message by omitting the provided Github token if present. */