Create GitHub Action for pylint script. (#9818)

* Create GitHub Action for pylint script.

* Fix indent

* Add back incremental flag from TF, remove allowlist.

Running a full-repo pylint with Github Actions CI takes >40 minutes,
too long for use on presubmit.

Allowlist is per-file, so the TF allowlist offers nothing here.

* Fetch master so our script has something to diff against.

* Checkout as branch rather than as merge.

This should (?) get us only our modified files.

* Diff to common ancestor.

* Terrible no-good commit introducing linter error.

* Point to locally fetched master.

* debug logging

* HEAD missing?

* Fetch full branches.

* Fetch master again.

* Revert "debug logging"

This reverts commit 6edde819537a162a2fe4010b0e66c82b088ea7a9.

* Revert "Terrible no-good commit introducing linter error."

This reverts commit ac41b830342a21a1711a9d77a11777276602fb96.