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

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.

.github/workflows/build.yml

@@ -32,7 +32,20 @@ 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 than git diff "origin/$GITHUB_BASE_REF.." (2 dots),
+            # but it requires to download more commits (this job uses
+            # "git fetch --depth=1").
+            #
+            # git diff "origin/$GITHUB_BASE_REF..." (3 dots) works with Git
+            # 2.26, but Git 2.28 is stricter and fails with "no merge base".
+            #
+            # 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)'