[MERGE] reject reserved revision ids

[Martin Pool]

Martin Pool has voted +1 (conditional).
Status is now: Conditionally approved
Comment:
I'm in favour of the checks.  I have some stylistic comments.

I would suggest that method names should generally have verbs in them -- 
so is_reserved_revision_id.  But beyond that, DRY suggests that since 
every call to this is just raising the same exception if it's wrong, why 
not put that in and call it check_for_reserved_revision_id (or some 
shorter variant)?

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C45A674C0.9040608%40utoronto.ca%3E