repo: 修改 REPO_URL

解决repo init失败的问题

if not REPO_URL:
  REPO_URL = 'https://gerrit.googlesource.com/git-repo'
REPO_REV = 'stable'

改为:

if not REPO_URL:
  REPO_URL = 'https://mirrors.tuna.tsinghua.edu.cn/git/git-repo/'
REPO_REV = 'stable'