Issue #29798: Handle git worktree in patchcheck (#1057)

The original attempted fix missed an `isdir()` call in
`get_base_branch()`.
diff --git a/Tools/scripts/patchcheck.py b/Tools/scripts/patchcheck.py
index 9f7fb19..be46fe3 100755
--- a/Tools/scripts/patchcheck.py
+++ b/Tools/scripts/patchcheck.py
@@ -74,7 +74,7 @@
 @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