Make lintrunner work with new main branch (#99466)
We've renamed the `master` branch to `main`. Lintrunner should check for a merge base from this new branch now
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### <samp>🤖 Generated by Copilot at 9743d70</samp>
Updated the linter configuration to reflect the new default branch name. Changed `merge_base_with` from `origin/master` to `origin/main` in `.lintrunner.toml`.
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### <samp>🤖 Generated by Copilot at 9743d70</samp>
> _`merge_base_with` changed_
> _`origin/main` is the new_
> _branch name for pytorch_
Pull Request resolved: https://github.com/pytorch/pytorch/pull/99466
Approved by: https://github.com/kit1980, https://github.com/malfet
