dispatch: show deprecation warning if command has no attributes (issue5137)
norepo/optionalrepo/inferrepo were removed by aa73d6a5d9ea, which would be
significant API change. This patch tries to avoid crash even if ancient
third-party extensions are enabled.
dispatch: extract function that tests command attributes
This function will host the compatibility layer for old third-party commands.
See the next patch for details.