Fix "Check for source changes (pull_request)" GH Action job

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.
python · vstinner · Aug 10, 2020 · Aug 10, 2020 · Aug 10, 2020 · Aug 10, 2020
commit 9a4c8ba2afb26375ab5aff3a287bcb2cd519c5cd
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
@@ -32,7 +32,15 @@ jobs:
             echo '::set-output name=run_tests::true'
           else
             git fetch origin $GITHUB_BASE_REF --depth=1
-            git diff --name-only origin/$GITHUB_BASE_REF... | grep -qvE '(\.rst$|^Doc|^Misc)' && echo '::set-output name=run_tests::true' || true
+            # git diff "origin/$GITHUB_BASE_REF..." (3 dots) may be more reliable,
+            # but it requires to download more commits.
+            #
+            # git diff "origin/$GITHUB_BASE_REF.." (2 dots) should be enough on
+            # GitHub, since GitHub starts by merging origin/$GITHUB_BASE_REF
+            # into the PR branch anyway.
+            #
+            # https://github.com/python/core-workflow/issues/373
+            git diff --name-only origin/$GITHUB_BASE_REF.. | grep -qvE '(\.rst$|^Doc|^Misc)' && echo '::set-output name=run_tests::true' || true
           fi
   build_win32:
     name: 'Windows (x86)'