Byteify registered command help and the `buglink` attribute
I hit this in various test-check-* tests for Mercurial when running on a zstd repo without the C extensions, but this can be trivially reproduced with `HGPLAIN=1`: *** failed to import extension mercurial_keyring: unicode 'keyring_check [PATH]' found in cmdtable.keyring_check *** (use b'' to make it byte string) The issue is that `i18n._()` wants bytes, but leaves a backdoor for `str` for docstrings. That backdoor is closed if `HGPLAIN` or `HGPLAINEXCEPT` is set (unless `i18n` is set in `HGPLAINEXCEPT`). There are a bunch of other uses of this function that still need to be byteified. The global `buglink`, `testedwith`, and `minimumhgversion` also want to be bytes.