[MERGE][Trivial] invalid version tuple attempt to raise AssertionError fails with TypeError

[Mark Hammond]

> Mark Hammond wrote:
> > bzrtools:
> >
> >  from bzrlib import revision as _mod_revision
> 
> That is not an example of importing a private symbol.

Could you please clarify?  It has a leading underscore, which is a
bright-red flag to my (and grep's ;) eyes...

> Anyhow, we want to provide a great API, and if you have to use a private 
> symbol, we're failing at that.  Please help us make our API great.

I tried to use an internal API function, and when I did it appeared to break
in a fairly fundamental way.  I diagnosed the problem and submitted a patch.
In response to a suggestion that I propose the API as being public, I
pointed out that many more mature plugins don't use the symbol, so maybe its
not that suitable for becoming public, and these plugins *do* use other
private symbols, and have done so for far longer than for my code has
existed.

I think I've done my duty :)

Martin: thanks for agreeing to merge it...

Cheers,

Mark