Gracefully handle PermissionError exceptions that crash fuzzer

Fuzzing inputs sometimes produce directory paths that are protected
inside the fuzzer execution environment. This is not an issue in
GitPython's code, so it should not crash the fuzzer.

Fixes OSS-Fuzz Issue 69456:
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=69870

fuzzing/fuzz-targets/fuzz_submodule.py

@@ -78,6 +78,7 @@ def TestOneInput(data):
             IsADirectoryError,
             NotADirectoryError,
             BrokenPipeError,
+            PermissionError,
         ):
             return -1
         except Exception as e: