bpo-29798: Handle git worktree in patchcheck (#1058) (#1060)

The original attempted fix missed an `isdir()` call in
`get_base_branch()`.
(cherry picked from commit 2abfdf5a81383d3b1ed6b7321903a9a168c373c5)
diff --git a/Tools/scripts/patchcheck.py b/Tools/scripts/patchcheck.py
index f4ec7d8..33a9fea 100755
--- a/Tools/scripts/patchcheck.py
+++ b/Tools/scripts/patchcheck.py
@@ -70,7 +70,7 @@
 @status("Getting base branch for PR",
         info=lambda x: x if x is not None else "not a PR branch")
 def get_base_branch():
-    if not os.path.isdir(os.path.join(SRCDIR, '.git')):
+    if not os.path.exists(os.path.join(SRCDIR, '.git')):
         # Not a git checkout, so there's no base branch
         return None
     version = sys.version_info