gh-109408: Run patchcheck in GitHub Actions

.github/workflows/lint.yml

@@ -24,3 +24,22 @@ jobs:
         with:
           python-version: "3.x"
       - uses: pre-commit/action@v3.0.0
+
+  patchcheck:
+    if: "github.repository == 'python/cpython' && github.event_name == 'pull_request'"
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+          ref: ${{ github.event.pull_request.head.ref }}
+          repository: ${{ github.event.pull_request.head.repo.full_name }}
+      - uses: actions/setup-python@v4
+        with:
+          python-version: "3.11"
+      - name: "Run patchcheck"
+        run: |
+          git status && git branch
+          python Tools/patchcheck/patchcheck.py --ci true

Tools/patchcheck/patchcheck.py

@@ -99,6 +99,7 @@ def get_git_remote_default_branch(remote_name):
         info=lambda x: x if x is not None else "not a PR branch")
 def get_base_branch():
     if not os.path.exists(os.path.join(SRCDIR, '.git')):
+        print('No', SRCDIR)
         # Not a git checkout, so there's no base branch
         return None
     upstream_remote = get_git_upstream_remote()
@@ -108,6 +109,7 @@ def get_base_branch():
     else:
         base_branch = "{0.major}.{0.minor}".format(version)
     this_branch = get_git_branch()
+    print(locals())
     if this_branch is None or this_branch == base_branch:
         # Not on a git PR branch, so there's no base branch
         return None