bpo-40548: Fix "Check for source changes (pull_request)" GH Action job (GH-21806) (GH-92342)

On Git 2.28, "git diff master..." (3 dots) no longer works when
"fetch --depth=1" is used, whereas it works on Git 2.26.

Replace "..." (3 dots) with ".." (2 dots) in the "git diff" command
computing the list of modified files between the base branch and the
PR branch.
(cherry picked from commit eaa551702d80fd67219c48ee6a13ffb571ca360b)

Co-authored-by: Victor Stinner <vstinner@python.org>