[MERGE] reject reserved revision ids

[Aaron Bentley]

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Martin Pool wrote:
> 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)?

Okay.  Thinking about this raised an interesting issue: do we want to
draw a distinction between version ids and revision ids?

That is, in versionedfiles, the ids are version ids, but when retrieving
revisions or revision trees, the ids are revision ids.  They happen to
have the same set of names, but Tom Lord would call this "punning", I
think, implying that they are the same by choice not by necessity.

Aaron
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