exthelper: support the option argument when registering a command
Largefiles uses this 5th argument with push and pull, so this will be tested in
the next commit. I assume the reason for unrolling and reforming the tuple in
each finalxxxsetup() is to validate that something proper was passed in when
registering. But it's better to explode when decorating than during the delayed
actual registration.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).