[GHF] Add submodule updates check (#95885)
Originally planned to integrate it somehow into the `lintrunner`, but this poses too many challenges, one of them is that it deliberately ignores submodule updates.
On the other hand, almost all the information, other than list of the submodules is already present in the GitHubPR info.
Incorporate small BE change into `test_trymerge.py`, that moves `@mock.patch` from individual test to the class definition.
Fixes https://github.com/pytorch/pytorch/issues/74326 and https://github.com/pytorch/test-infra/issues/1521
Pull Request resolved: https://github.com/pytorch/pytorch/pull/95885
Approved by: https://github.com/ZainRizvi, https://github.com/huydhn
