Use remote master as the linter merge base (#97800)
Fixes https://github.com/pytorch/pytorch/issues/96794
Sometimes people never update their local `master` branch. Their workflow instead consists of fetching commits from git and directly creating branches off of the remote `master` branch (e.g. via `git checkout -b <mybranch> origin/master`
For those people, their local `master` is very old and out of date, creating an unreasonably old lint base that tends to catch all sorts of unrelated linter errors.
Anyone with an updated `master` branch will naturally have an updated pointer to the remote `master`, so this change makes lintrunner friendly to both behavior patterns
Pull Request resolved: https://github.com/pytorch/pytorch/pull/97800
Approved by: https://github.com/huydhn
