gendoc: guarantee that all commands were processed
The new logic renders the commands belonging to each category in turn.
Commands with an unregistered category are at risk of getting skipped
because their category is not in the list. By comparing the list of all
commands to a log of processed commands, we can detect commands with
unregistered categories and fail with an error message.
Differential Revision: https://phab.mercurial-scm.org/D6327