`envoy.code.check --since` can fail

Not sure why it does/not work locally, but testing ...

```console

$ bazel run //tools/code:check -- -s main

File "/home/worker/.cache/bazel/_bazel_worker/f704bab1b165ed1368cb88f9f49e7532/execroot/envoy/bazel-out/k8-fastbuild/bin/tools/code/check.runfiles/base_pip3_aio_core/site-packages/aio/core/functional/decorators.py", line 128, in load_cooperatively
return await self.load(instance)
File "/home/worker/.cache/bazel/_bazel_worker/f704bab1b165ed1368cb88f9f49e7532/execroot/envoy/bazel-out/k8-fastbuild/bin/tools/code/check.runfiles/base_pip3_aio_core/site-packages/aio/core/dev/debug.py", line 89, in fun_async
await self.__wrapped__(*args, **kwargs))
File "/home/worker/.cache/bazel/_bazel_worker/f704bab1b165ed1368cb88f9f49e7532/execroot/envoy/bazel-out/k8-fastbuild/bin/tools/code/check.runfiles/base_pip3_aio_core/site-packages/aio/core/functional/decorators.py", line 120, in load
return self.set_prop_cache(instance, await self.fun(instance))
File "/home/worker/.cache/bazel/_bazel_worker/f704bab1b165ed1368cb88f9f49e7532/execroot/envoy/bazel-out/k8-fastbuild/bin/tools/code/check.runfiles/base_pip3_aio_core/site-packages/aio/core/directory/abstract/directory.py", line 371, in files
(await self.get_files()
TypeError: unsupported operand type(s) for &: 'set' and 'dict'

```

whereas ` bazel run //tools/code:check -- -s upstream/main` works - i guess because these produce different filesets