Use os.path.normcase in git-sync-deps

Avoid strange issues with path case insensitivity on Windows with
os.path.normcase.

Change-Id: I9327ba9c22cf5e3ff66b49af56bedd223b24efd4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/556025
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>

tools/git-sync-deps

@@ -94,7 +94,8 @@ def is_git_toplevel(git, directory):
   try:
     toplevel = subprocess.check_output(
       [git, 'rev-parse', '--show-toplevel'], cwd=directory).strip()
-    return os.path.realpath(directory) == os.path.realpath(toplevel.decode())
+    return (os.path.normcase(os.path.realpath(directory)) ==
+            os.path.normcase(os.path.realpath(toplevel.decode())))
   except subprocess.CalledProcessError:
     return False